Nullify, cancel out (6)
I believe the answer is:
negate
This clue is a double definition.
'nullify' is the first definition.
(I know that nullify is a kind of negate)
'cancel out' is the second definition.
(I've seen this in another clue)
(Other definitions for negate that I've seen before include "Cancel; invalidate" , "Disprove or nullify" , "Render null or disprove" , "Nullify - deny the existence of" , "disallow!" .)