Unit of speed; one nautical mile per hour (4)
I believe the answer is:
knot
I believe this is a double definition.
'unit of speed' is the first definition.
(I've seen this in another clue)
'one nautical mile per hour' is the second definition.
(I've seen this in another clue)
(Other definitions for knot that I've seen before include "Measure of speed of, for example, frigate" , "Fastener - bird - unit of speed at sea" , "Shore bird; cluster" , "Air mile" , "1 (nautical) mph" .)