Unit of nautical speed (4)
I believe the answer is:
knot
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unit of nautical speed</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)