Thwart or frustrate (4)
I believe the answer is:
foil
I believe this is a double definition.
'thwart' is the first definition.
(I know that thwart can be written as foil)
'frustrate' is the second definition.
(I know that frustrate can be written as foil)
(Other definitions for foil that I've seen before include "Fencing weapon; outwit" , "Hinder - prevent" , "Prevent from succeeding" , "Frustrate; kitchen wrap" , "Frustrate with fencing sword" .)