He wins at a single stroke (3)
I believe the answer is:
ace
'stroke' is the definition.
(ace is a kind of stroke)
'he wins at a single' is the wordplay.
'he wins' becomes 'ce' (I am not sure about this - if you are sure you should believe this answer much more).
'at' is a charade indicator (letters next to each other) (I've seen this in other clues).
'a single' becomes 'a'.
'ce' put after 'a' is 'ACE'.
Can you help me to learn more?
(Other definitions for ace that I've seen before include "winner in court" , "Crack" , "smashing" , "'wicked'!" , "Top playing card" .)