Undo a knot (5)
I believe the answer is:
untie
'undo a knot' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for untie that I've seen before include "Remove ropes, unfasten" , "Loosen ropes or knots" , "Unite (anag)" , "Detach" , "resolve" .)