Release, unfasten (5)
I believe the answer is:
untie
I believe this clue is a double definition.
'release' is the first definition.
(eg to untie restraints)
'unfasten' is the second definition.
(I know that untie is a more specific form of the action the unfasten)
(Other definitions for untie that I've seen before include "Open a knot" , "Unite (anag)" , "Undo, unfasten" , "Undo a knot" , "get rid of bonds" .)