One nautical mile per hour (4)
I believe the answer is:
knot
'one nautical mile per hour' 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 "Ravel" , "Unit of nautical speed" , "A sheepshank or bowline" , "measure of pace" , "Granny or reef, perhaps?" .)