Prove wrong (8)
I believe the answer is:
confound
'prove wrong' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for confound that I've seen before include "Thwart - baffle" , "Puzzle" , "Befuddle, confuse" , "Defeat (a plan or hope)" , "Surprise; defeat (plan)" .)