American nut in the lead (5)
I believe the answer is:
ahead
'in the lead' is the definition.
(synonyms)
'american nut' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'nut' becomes 'head' (nut can mean one's head).
'a'+'head'='AHEAD'
(Other definitions for ahead that I've seen before include "likely to win?" , "In the lead, in advance" , "In front as in race" , "in van" , "Winning" .)