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

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?