Ace captain, winning (5)
I believe the answer is:
ahead
'winning' is the definition.
(both can mean in the lead)
'ace captain' is the wordplay.
'ace' becomes 'a'.
'captain' becomes 'head' (both can mean a leader).
'a'+'head'='AHEAD'
(Other definitions for ahead that I've seen before include "likely to win?" , "advanced" , "In the vanguard" , "in van" , "What's down the road" .)