Nautical unit of speed (4)
I believe the answer is:
'nautical unit of speed' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for knot that I've seen before include "Air mile" , "Small cluster - huddled group - sheepshank, for example" , "Tangled mass" , "Granny or reef, perhaps?" , "Ring < sandpiper" .)