Champion with secret (6)
I believe the answer is:
winner
'champion' is the definition.
(winner of a competition)
'with secret' is the wordplay.
'with' becomes 'w' (abbreviation).
'secret' becomes 'inner' (as in one's inner thoughts).
'w'+'inner'='WINNER'
(Other definitions for winner that I've seen before include "Victor" , "success" , "The champ" , "film director" , "Champion" .)