Undoes a knot (6)
I believe the answer is:
unties
'undoes a knot' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for unties that I've seen before include "Loosens laces" , "Undoes, a knot perhaps" , "Undoes, loosens a knot" , "Opens knot to release" .)