Fathomless pit (5)
I believe the answer is:
abyss
'fathomless pit' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for abyss that I've seen before include "Hell; hole" , "Primal chaos" , "Seemingly bottomless chasm" , "Deep gulf" , "Fathomless pit" .)