Reverse defeat (4)
I believe the answer is:
undo
I believe this is a double definition.
'reverse' is the first definition.
(I know this)
'defeat' is the second definition.
(thesaurus)
(Other definitions for undo that I've seen before include "Ruin or loosen ties" , "Bring to ruin" , "Dissolve - ruin" , "Subvert" , "Untie, loosen" .)