Disfigure agent that's turned over winning card (6)
I believe the answer is:
deface
'disfigure' is the definition.
(I know that disfigure can be written as deface)
'agent that's turned over winning card' is the wordplay.
'agent' becomes 'fed' (fed is a kind of agent).
'that's turned over' is a reversal indicator.
'winning card' becomes 'ace' (I've seen this before).
'fed' backwards is 'def'.
'def'+'ace'='DEFACE'
(Other definitions for deface that I've seen before include "Disfigure, spoil the surface of" , "Spoil or damage the surface of" , "six notes?" , "Spoil surface of, mar" , "Vandalise" .)