It's a one-stroke (3)
I believe the answer is:
ace
I believe this 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 "fighter pilot" , "Brilliant" , "Outstanding" , "In golf, a hole in one" , "Adept" .)