Six feet of water (6)
I believe the answer is:
fathom
'six feet of water' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for fathom that I've seen before include "Measure of depth, 6 ft." , "Six-foot measure" , "Unit of measurement for water depth" , "Nautical depth of six feet" , "It's six feet deep" .)