Does the Gloom Stalker ranger's Dread Ambusher feature apply only when the party actually engages in combat, or when initiative is rolled?

Question

The Gloom Stalker ranger's Dread Ambusher feature says, in part (XGtE, p. 42; emphasis mine):

At the start of your first turn of each combat, your walking speed increases by 10 feet, which lasts until the end of that turn. If you take the Attack action on that turn, you can make one additional weapon attack as part of that action. If that attack hits, the target takes an extra 1d8 damage of the weapon's damage type

If the DM calls for initiative to be rolled earlier than the party could reasonably engage in "combat" (such as a combat happening at the gates of a city far away, or off in the distance), can the ranger still benefit from Dread Ambusher once actual combat starts for the party/ranger?

The class feature itself doesn't specify that the ranger needs cover, or surprise, and it doesn't specifically say that the class feature starts on "initiative" but rather on the first turn of combat.

Or is the benefit of Dread Ambusher just gone if the DM decides to call for initiative more than a round before the ranger can engage in combat?

Answer 1

Initiative is rolled when combat starts

Per the rule for Initiative:

When combat starts, every participant makes a Dexterity check to determine their place in the initiative order.

If combat starts but you're out of range to attack, then RAW, that has no bearing on the effects of Dread Ambusher.

If you feel that doing so is denying you the usage of your class features, you should work with your DM about when the start of combat should be called.

What if the Ranger joins combat late?

(Thanks for SeriousBri in the comments for clarifying the question here!)

Only "participants" roll initiative when combat starts, and the rules are silent on how to add participants to a combat already in progress. Doing so is up to the DM's adjudication - so talk to your DM about what Dread Ambusher means in this context.

At my table

Speaking for myself, I allow PCs to remain "not in combat" as long as they're not taking combat actions and no enemies recognize them as a hostile combatant. They would be allowed to enter the battle at the start of each combat round if they choose to or are forced to participate, such as by being spotted by an enemy. That is heavily imperfect and leads to all sorts of edge cases (what if they get spotted them in the middle of a combat round? What if they get caught in the AOE of a Fireball?) which I'd rule on a case-by-case basis.

My preference as a DM is to avoid those edge-cases by avoiding the situation in the first place. The rules work best if combat "starts" when the PCs become involved in it (a combat between NPCs does not use the combat rules; it just gets narrated), and if all PCs that are going to be involved in a combat enter are involved from the start.