Hard to fathom (4)
I believe the answer is:
deep
'hard to fathom?' is the definition.
(to fathom is to measure a body of water's depth)
This is all the clue.
(Other definitions for deep that I've seen before include "(Of sound) low in pitch" , "Intense -- extreme" , "down a long way" , "Extending far down" , "Heartfelt" .)