Moves reflect things that happen in the fiction. It's important to keep your fiction synced with your players'.
So, one of the important things about getting to grips with any Apocalypse Engine game in general, and Dungeon World in specific, is to internalize that there is no seperate mechanical action called "making a move".
On the player end, it's saying something that prompts a die roll or a structured GM response. On the GM end, it's something that prompts a player response. But before anybody starts resolving a move, it's important that everyone be clear on what's happening in the fiction.
What the GM Should Say
So, right, you've got this mental image of HILL GIANT using CLUB on FIGHTGAR, but you don't say that. Instead:
Start from the idea of one of your GM moves, which is probably "put someone in a spot" given your stated intentions; basically, give them something dangerous to react to.
Look at what you want to use to make it and work out how that's going to happen: HILL GIANT used CLUB, that's easy enough.
Here's where you start actually talking. Describe the things that are happening as clearly as you can. How is HILL GIANT using CLUB: at the edge of its reach, right up close, flailing with one hand, securely with two hands, underhand, overhand, sweeping? Is anything else going on while that's happening: lesser enemies fleeing, the ground shaking, the swing hits some terrain or structure and brings debris along with?
Okay, now you tag to Fightgar. Fightgar, what are you doing?
In all likelihood I am making this seem more complicated than it usually winds up being. This is the full debug procedure; often you'll "start at step 2" with an idea of what you want to happen, so just quickly look back at step 1 to make sure you're giving players something to react to. Like, you're looking to put Fightgar in a spot, so make sure when you describe the swing you tell Fightgar why it's putting them in danger. Probably it's headed right for them, but it could also be aimed at taking out something that Fightgar's inside of, on top of, maybe just in front of.
And then you look at Fightgar's player, who for purposes of this example we will call Steve, because it's Steve's turn to talk.
What the GM Should Say to Steve
There's one important difference here, right up front. When you told the table what the hill giant was doing, you didn't mention any of the GM moves you were using, or any of the hill giant's monster moves for that matter. You just talked about what was happening. For you, moves are there as a starting point to help think about what's happening. Actually telling people the names doesn't really give them new information.
Player-facing moves, on the other hand, have a bunch of scripting attached to them, and as a result everybody needs to agree that the move can happen, and be on the same page as to how it's happening, before the script can kick in. For that reason, bringing up move names as part of that discussion is totally fine.
So if Steve says Fightgar is going to Hack And Slash the giant, and you agree that Fightgar can currently just engage the giant in melee, you still need to know how Steve thinks that's happening. The canonical means of doing this is to say "Cool, how?" until you're satisfied, but after that Steve can just roll Hack And Slash and you can go from there.
Or maybe you don't agree, in which case explain why, and offer a path to get there if you can. Like:
You're 30 feet away and that club's coming in awful fast. You're going to have to Defy Danger to get close enough to hit the giant without being batted away.
And if Steve doesn't roll Defy Danger and just says Fightgar runs screaming at the giant, perhaps because it's what Fightgar would do, well, Fightgar gets batted away before they get close, with damage as you deem appropriate, and Steve doesn't get to roll Hack And Slash at all. It's what you said would happen. (This is called "the player giving you a golden opportunity".)
But why would you agree, or not agree, that Fightgar can currently engage the giant in melee?
What Melee Is, Or, Why Defy Danger Is Kind Of Rubbish As A Move
Simply put, melee is a thing where all the parties are engaged in close combat, capable of hurting each other and being hurt, with no particular advantage to one side or another.
Is Fightgar, at the edge of the giant's reach, as much a danger to the giant just then as the giant is to Fightgar?
And if not, at what point does that happen?
These are things you will have to decide for yourself, based on the further facts of Fightgar and of the giant that may exist.
Maybe Fightgar's signature weapon is a weighty stone hammer, renowned as "the Giant-Killer", and they know well enough how to fight giants that they are a danger to giants at any range.
Maybe the giant is wearing tall iron "siege boots", or has trained in fighting "the little people" so extensively that Fightgar will need to use terrain or trickery or teamwork to even set up to land a solid blow.
Maybe it's somewhere in between - once Fightgar can get inside the reach of that club, they can cut and slice at whatever bits of the giant are close, and the giant can stomp and slap and grab at Fightgar.
But the thing you really should remember, the thing a lot of people often lose track of in the heat of the moment, is that combat is dangerous. Heck, lots of things are dangerous! Very few of the moves on offer in the book assume they're being done in circumstances of absolute safety.
Defy Danger seems like it could apply all the time, and it could apply all the time, but more often than not the dangers are wrapped up in the fallout of some other move. Defy Danger is for unexpected, unusual, or overwhelming dangers that can't be dealt with any other way. So even if Fightgar needs to Defy Danger to try and get close to the giant, if they can engage in melee normally then keep to that. Don't get into a loop where first Fightgar has to keep their balance when the giant stomps and then has to Hack and Slash, or has to dodge away from the giant's grab and then has to Hack and Slash. Only pull that out if you want to make fighting the giant, even up close, significantly more dangerous than any other fight.
Addendum: The Doink Imperative
So let's suppose that the giant is wearing high iron siege boots but is wading through fog so they're not visible, or is trained to pound on littlies but it isn't immediately obvious to Fightgar that's the case, and Fightgar rushes in like it's any other fight.
So, how do you say that without revealing something that Fightgar shouldn't know?
Well, even if Fightgar can't deal damage, Hack and Slash can still come into play here because you want to know how well Fightgar can avoid getting hit in melee combat, and Hack and Slash covers that pretty well too. On a "hit", Fightgar will know for certain they can't just get into melee with their target, whatever else happens.