Unit of nautical speed (4)
I believe the answer is:
knot
'unit of nautical speed' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for knot that I've seen before include "Hard mass in tree trunk at branch joint" , "1 (nautical) mph" , "Sort of garden" , "Unit of nautical speed" , "measure of pace" .)