Overturn, revoke (6)
I believe the answer is:
repeal
This clue is a double definition.
'overturn' is the first definition.
(I know that overturn can be written as repeal)
'revoke' is the second definition.
(I know that revoke can be written as repeal)
(Other definitions for repeal that I've seen before include "reverse" , "Annul a law" , "Cancel or nullify a law" , "Rescind or withdraw a law" , "Annul by legislative action" .)