It could be hell! (5)
I believe the answer is:
abyss
'be hell' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for abyss that I've seen before include "Deep gulf" , "Primal chaos" , "Measureless chasm" , "great depth" , "Hell; hole" .)