Loosen (a knot) (5)
I believe the answer is:
untie
'loosen a knot' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for untie that I've seen before include "Release, loosen" , "Undo a knot" , "Let loose" , "Free from binds" , "Detach" .)