Ace boss is winning (5)
I believe the answer is:
ahead
'is winning' is the definition.
(both can mean in the lead)
'ace boss' is the wordplay.
'ace' becomes 'a' (abbreviation).
'boss' becomes 'head' (synonyms).
'a'+'head'='AHEAD'
(Other definitions for ahead that I've seen before include "in van" , "at pole" , "Forwards" , "Winning" , "advanced" .)