Fathom (5)
I believe the answer is:
plumb
'fathom' is the definition.
(I've seen this before)
This is all the clue.
(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" .)