Cancel; revoke (5)
I believe the answer is:
annul
This clue is a double definition.
'cancel' is the first definition.
(I know that annul is a type of cancel)
'revoke' is the second definition.
(I know that revoke can be written as annul)
(Other definitions for annul that I've seen before include "Repeal, rescind" , "Cancel officially" , "Scrap" , "Declare (a law) invalid" , "Countermand" .)