Unit of speed; one nautical mile per hour (4)
I believe the answer is:
knot
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">unit of speed</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >'<span class="explanation-format__original">one nautical mile per hour</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(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" .)