A capital forward (5)
I believe the answer is:
ahead
'forward' is the definition.
(synonyms)
'a capital' is the wordplay.
'capital' becomes 'head' ().
'a'+'head'='AHEAD'
(Other definitions for ahead that I've seen before include "In the vanguard" , "Forward - in front" , "Winning" , "in van" , "In the leading position" .)