Fool eating next to very deep hole (5)
I believe the answer is:
abyss
'very deep hole' is the definition.
'fool eating next to' is the wordplay.
'fool' becomes 'ass' (ass is a kind of fool).
'eating' is an insertion indicator.
'next to' becomes 'by'.
'ass' placed around 'by' is 'ABYSS'.
(Other definitions for abyss that I've seen before include "Bottomless pit" , "great depth" , "Hell; hole" , "Deep gulf" , "gaping hole" .)