Unit of depth (6)
I believe the answer is:
fathom
'unit of depth' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)