Challenge win, for example (7)
I believe the answer is:
gainsay
'challenge' is the definition.
('gainsay' can be a synonym of 'challenge')
'win for example' is the wordplay.
'win' becomes 'gain' ('gain' can be a synonym of 'win').
'for example' becomes 'say' (I've seen this before).
'gain'+'say'='GAINSAY'
(Other definitions for gainsay that I've seen before include "Speak against" , "Challenge, dispute" , "Contradict, deny, dispute" .)