Loosen, as a knot (4)
I believe the answer is:
undo
'loosen as a knot' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for undo that I've seen before include "Loosen or reverse" , "Relax" , "Ruin prospects of; cancel" , "Destroy - nullify" , "Ruin or loosen ties" .)