Leading article — six Times rejected (5)
I believe the answer is:
'leading' is the definition.
('ahead' can be a synonym of 'leading')
'article six times rejected' is the wordplay.
'article' becomes 'a' (indefinite article).
'six' becomes 'hexad' ('hexad' can be a synonym of 'six').
'times' becomes 'x' (multiplication sign).
'rejected' indicates named letters should be taken away.
'hexad' with 'x' taken away is 'head'.
(Other definitions for ahead that I've seen before include "at pole" , "in van" , "What's down the road" , "likely to win?" , "In the future" .)