There isn't "combat". There's just "monsters trying to hurt PCs."
I mean, there is combat-the-fictional-thing, if you want to define that as, for example, "a fight to the death between PCs and monsters, with neither side holding an overwhelming advantage". Some playbooks care about going into combat or limiting use of features per combat. But from a GM's point of view, there's nothing special about that state of affairs as compared to any other, no difference in the pace of play, no extra responsibilities (and no fewer either). There's no combat-the-game-state, in other words.
You might be thinking about a fairly common scenario that, if this were a video game, would be "the monsters strike first!", where the monsters pull PCs into combat-the-fictional-thing at a disadvantage. This is a sensible desire, especially when, y'know, Fightgar is delving deep into the Ruins of the Ambush Goblins, and what's going to happen, not ambushes?
The Ambush: Starting with Disadvantage
However, keep in mind that you're there to be a fan of the players and fill their lives with adventure. Starting from a neutral position, just straight-up dropping damage on somebody isn't very adventurous and doesn't contain much to be a fan of -- and you've already got that impulse, right? It feels like there should be something that determines whether or not "the attack is successful", that you should be able to go, like--
Your torch creates patterns of light and shadow on the arched ceiling as you press forward, Fightgar. Suddenly, there's a movement, and you spin and raise your torch just in time to see a goblin leaping from a ceiling nook, brandishing a wicked-looking knife. "Glory in shadow!" it screams. You only have a split-second to react. What are you doing?
And this is also a little more adventurous, isn't it? Fightgar's on the back foot! What's going to happen?
But because Fightgar's at a disadvantage, you should also feel free to tell them the requirements or consequences and then ask if they try and do something that assumes they're on an even keel - sure, you can draw Endbringer and attack the goblin in melee, but the goblin's got the drop on you, literally, and you will take at least one stab in addition to whatever results from your hack and slash.
Probably the only move that won't have this sort of rider is defy danger -- not because it's the "a monster is attacking you, dodge" move, but because it's useful for moving out of a temporary disadvantage state and back to even. You shouldn't be regularly calling for defy danger during combats - if Fightgar is an even match for what they're fighting, it's likely to stay that way.
It is completely legitimate for you do this from a neutral setup.
This is a move that puts Fightgar in a spot, which is a perfectly fine thing for a move to do, and you can make a move any time the players look to you to find out what happens. Who at the table knows what will happen when Fightgar delves deeper into the Ruins of the Ambush Goblins? There are probably some justified suspicions out there, but the only one who knows for sure is you, the GM. Your free choice of moves when the conversation turns back to you should be tempered by the principles you're trying to follow.
(This is where things fall apart a little if you're trying to run Dungeon World as a solo adventuring engine where you are both player and GM - deciding where to draw the line about what player-you knows. It's not easy, but trusting your instincts about when the conversation should pass from one person to another is a good guiding principle. You have a lot more experience having conversations than you do playing Dungeon World.)
Of course, if it's not a neutral setup -- if, for instance, Fightgar wisely attempted to Discern some Realities about the Ruins of the Ambush Goblins but unwisely rolled a 4 -- then you have a little more leeway to have the monsters start out successful:
What should you be on the lookout for, Fightgar? Well, you sweep the torch around as you walk and -- oh, wow. Those aren't just shadows behind the pillars and the ceiling trim. This whole place is shot through with tunnels! How deep do they run? What's in them? Oh gods they could come from anywhere. It's almost a relief when a goblin takes a flying leap from the ceiling screaming "Glory in shadow!" and clamps onto your torso. The wicked-looking knife that sinks into your shoulderblade is less of a relief -- take d8 damage, piercing 1. Oh, and you've got a goblin clamped onto your torso ready to do the same thing again. What are you doing?
I've been using just plain ol' deal damage in these examples, but keep in mind that you're also here to give every monster life:
Think about more than just the exchange of damage. Monsters might be trying to capture the characters or protect something from them. Understand what the fight is about; what each side wants and how that might affect the tide of battle.
"The GM -- Fights", from the repo here
So, what do the monsters want? "Blood and pain" is a fine answer, especially for, say, a starved ocelot or a wandering patrol of mindless undead or, yes, a goblin ambusher, none of whom are big-picture thinkers. But other monsters might want different things, as evidenced in their instinct or their moves, and so when they have the PCs at a disadvantage they might choose to do something different.
- A giant eagle is going to hunt by dashing you on the rocks, so its opening move is not to just hurt you, but to pull you into the air.
- A hobgoblin slavetaker wants to take slaves, so it'll manacle your arms or legs when it gets the chance.
- Ossian, the lich, wants to channel the power of the stellar convergence without interference, to keep on un-living, so he puts into motion the ritual of the Hundred Mirrors, separating you and your companions from each other, and from most of reality.
But none of these are that different in execution from dealing damage - if they're intended to catch the PCs off guard, then present them as an imminent threat the PCs must react to, or if the PCs gave you an opening, just make them happen.