Remove a knot from (5)
I believe the answer is:
untie
'remove a knot from' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for untie that I've seen before include "Disconnect; release" , "Open a knot" , "Loosen, as laces" , "resolve" , "Undo, unlace" .)