Cancel out, invalidate (7)
I believe the answer is:
nullify
This clue is a double definition.
'cancel out' is the first definition.
(I've seen this before)
'invalidate' is the second definition.
(I know that invalidate can be written as nullify)
(Other definitions for nullify that I've seen before include "Make of no effect" , "Neutralise" , "quash" , "Render ineffective" , "Declare invalid" .)