Comprehend six feet (6)
I believe the answer is:
fathom
I believe this is a double definition.
'comprehend' is the first definition.
(I've seen this before)
'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" .)