Nautical speed unit (4)
I believe the answer is:
knot
'nautical speed unit' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for knot that I've seen before include "Small sandpiper - speed at sea - hitch" , "Eskimo people" , "It may be tied" , "Ravel" , "Tangled mass" .)