Measure of speed (4)
I believe the answer is:
knot
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">measure of speed</span>' is the definition.<br/ ><span class="explanation-format__explanation">(nautical unit of speed)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for knot that I've seen before include "Unit of speed in water" , "Unit of speed used by ships" , "Ring < sandpiper" , "Granny or reef, perhaps?" , "Assemblage" .)