Difficult problem for Ravel (4)
I believe the answer is:
knot
I believe this clue is a double definition.
'difficult problem' is the first definition.
(I've seen this before)
'ravel' is the second definition.
(synonyms)
'for' is the link.
(Other definitions for knot that I've seen before include "Could be a granny or a sheepshank" , "Tie cord - lump in timber" , "Small sandpiper - speed at sea - hitch" , "Unit of speed in water" , "Measure of speed on water" .)