Unfasten, loosen (4)
I believe the answer is:
undo
This clue is a double definition.
'unfasten' is the first definition.
(I've seen this in another clue)
'loosen' is the second definition.
(I've seen this in another clue)
(Other definitions for undo that I've seen before include "Cause downfall" , "Cancel or reverse an action" , "Untie or ruin" , "Destroy - nullify" , "Free from ties, laces, etc" .)