Loosen link, it's back to front (5)
I believe the answer is:
untie
'loosen' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
(Other definitions for untie that I've seen before include "get rid of bonds" , "Take off fastenings" , "Free from restraining bonds or ropes" , "Set free" , "Remove ropes, unfasten" .)