Thwart, foil (9)
I believe the answer is:
frustrate
This clue is a double definition.
'thwart' is the first definition.
(I know that thwart can be written as frustrate)
'foil' is the second definition.
(I know that foil can be written as frustrate)
(Other definitions for frustrate that I've seen before include "Prevent (plan) from being fulfilled" , "'Thwart, disappoint (9)'" , "Prevent from progressing" , "Thwart or baffle" , "Put a stop to" .)