It's a one-stroke (3)
I believe the answer is:
Here is my best explanation:
This clue is a double definition.
'it's a one' is the first definition.
('ace' can be a synonym of 'one')
'stroke' is the second definition.
(ace is a kind of stroke)
(Other definitions for ace that I've seen before include "Playing card - expert", "Person who excels, a top card", "Playing card with single spot", "Single unit on playing cards", "One in a suit", "Tennis serve that the opponent can't reach", "One or 11 in cards", "Hole in one (golf)", "Good; service; card" and "One; expert".)