Nullify or disprove (6)
I believe the answer is:
negate
I believe this is a double definition.
'nullify' is the first definition.
(I know that nullify can be written as negate)
'disprove' is the second definition.
(I've seen this in another clue)
(Other definitions for negate that I've seen before include "Render ineffective" , "Countermand" , "Nullify - deny the existence of" , "Cancel; invalidate" , "Cause to have no effect" .)