Friend’s winning move (4)
I believe the answer is:
mate
I believe this is a double definition.
'friend's' is the first definition.
(mate is a kind of friend)
'winning move' is the second definition.
(checkmate in chess)
(Other definitions for mate that I've seen before include "Officer below master on a commercial ship" , "declared winning position on board" , "Spouse on team" , "Pal, chum" , "Friend; ship's officer" .)