Annul, revoke (6)
I believe the answer is:
cancel
I believe this clue is a double definition.
'annul' is the first definition.
(I've seen this in another clue)
'revoke' is the second definition.
(I've seen this before)
(Other definitions for cancel that I've seen before include "Annul, revoke" , "Scrub" , "Write off" , "Annul; negate" , "Officially prevent happening" .)