Loosen, as a bra (6)
I believe the answer is:
unhook
'loosen as a bra' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for unhook that I've seen before include "Release catch" , "Unfasten" , "Lift off the sharp bent line" , "Free" , "Detach" .)