Cancel — annul (4)
I believe this clue is a double definition.
'cancel' is the first definition.
(I've seen this in another clue)
'annul' is the second definition.
(I've seen this before)
(Other definitions for undo that I've seen before include "Wreck" , "Unfasten - destroy" , "Loosen; cancel" , "Ruin or loose" , "Cause to become loose" .)
This clue is a double definition.
'cancel' is the first definition.
(to rescind is to cancel or repeal)
'annul' is the second definition.
(I know that annul can be written as rescind)
(Other definitions for rescind that I've seen before include "Revoke (order)" , "Cancel (regulation)" , "Withdraw (decree)" , "undo" , "Scrub" .)