When Dungeon World cribs from Apocalypse World, look to Apocalypse World to explain.
This is especially obvious in the basic Defy Danger move, where it doesn't really spend the time to discuss the outcomes the way Apocalypse World does, but fortunately the word choice points the way back to Act Under Fire.
And hey, it does it again! Here's what happens if you stick around long enough to advance Act Under Fire, which this is also cribbing from:
On a 12+, you transcend the danger, the pressure, the possibility of harm. You do what you set out to do, and the MC will offer you a better outcome, true beauty, or a moment of grace.
Tall order! So much depends on the details of the character's actions and the circumstances. If you know what the character's hoping for beyond the immediate, you can give it to her, that's probably easiest. Another thing you can do is have what the player's character does affect the NPCs witnessing it -- they're struck, awed, moved, taken aback, left breathless.
-- AW2e, "Improvement", p.265
So if the Defy Danger was in service of something, let that succeed too, no rolling. (If it was a backstab, treat it like a 10+.)
If it wasn't, but the Thief's among friends and foes, they can pass the benefits of the distraction off to someone else.
If they aren't, and only foes are around, they're just watching in awe.
If they aren't, well, surely someone's around to see what the Thief just pulled and be impressed.
If no one is, how sure of that are you?
If you're sure, well, "you show off and feel fantastic, take +1 forward" isn't a bad consolation prize.