Difficult problem for Ravel (4)
I believe the answer is:
I believe this clue is a double definition.
'difficult problem' is the first definition.
(I've seen this before)
'ravel' is the second definition.
'for' is the link.
(Other definitions for knot that I've seen before include "Tangle; sandpiper", "It makes fast", "Unit of nautical speed", "Small northern sandpiper", "Unit of speed used by ships".)