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 "One; expert", "One or 11 in cards", "Person who excels, a top card", "Great", "Playing card with a single symbol", "Wicked", "One-spotted card", "Very good; playing card", "one among a set of diamonds" and "Girl".)