Comprehend six feet (6)
I believe the answer is:
fathom
This clue is a double definition.
'comprehend' is the first definition.
(I've seen this in another clue)
'six feet' is the second definition.
(I've seen this before)
(Other definitions for fathom that I've seen before include "Come to understand the water depth" , "Get to the bottom of" , "some distance down" , "Nautical depth of six feet" , "Comprehend - a depth of six feet" .)