Unit of depth (6)
I believe the answer is:
fathom
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unit of depth</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 fathom that I've seen before include "Unit of depth - comprehend it" , "Get to the bottom of" , "Discover the meaning of - six feet" , "Six-foot measure" , "Come to understand - unit of depth" .)