Open to defeat (4)
I believe the answer is:
undo
I believe this is a double definition.
'open' is the first definition.
(thesaurus)
'defeat' is the second definition.
(thesaurus)
'to' is the link.
(Other definitions for undo that I've seen before include "Cancel or reverse an action" , "Dissolve - ruin" , "Unfasten - destroy" , "Unravel or ruin" , "Ruin or undermine" .)