Advanced with leader in front (5)
I believe the answer is:
ahead
'in front' is the definition.
(synonyms)
'advanced with leader' is the wordplay.
'advanced' becomes 'a'.
'with' means one lot of letters go next to another.
'leader' becomes 'head' (head is a kind of leader).
'a'+'head'='AHEAD'
(Other definitions for ahead that I've seen before include "In front as in race" , "advanced" , "Forwards" , "Winning" , "at pole" .)