Undo, unlock (4)
I believe the answer is:
open
This clue is a double definition.
'undo' is the first definition.
(opening is a kind of undoing)
'unlock' is the second definition.
(I've seen this in another clue)
(Other definitions for open that I've seen before include "Affording access" , "simple" , "unsealed" , "Overt" , "Not shut up or hidden" .)