Fathom (5)
I believe the answer is:
plumb
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fathom</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for plumb that I've seen before include "Fathom - out-and-out" , "Weight to show vertical direction" , "True" , "Vertical; measure (depth)" , "Check depth of water below" .)