Cancel, invalidate (5)
I believe the answer is:
annul
I believe this is a double definition.
'cancel' is the first definition.
(I know that annul is a more specific form of the action cancel)
'invalidate' is the second definition.
(I know that invalidate can be written as annul)
(Other definitions for annul that I've seen before include "Declare (a law) invalid" , "Scrap" , "Cancel, say a law" , "Render invalid" , "scratch" .)