Fool eating next to very deep hole (5)
I believe the answer is:
Here is my best explanation:
'very deep hole' is the definition.
'fool eating next to' is the subsidiary indication.
'fool' becomes 'ass' (ass is a kind of fool).
'eating' is an insertion indicator.
'next to' becomes 'by'.
'ass' going around 'by' is 'abyss'.
(Other definitions for abyss that I've seen before include "The depths", "Very deep pit", "Hell; hole", "depths of sea", "Gulf", "gaping hole", "Deep hole, seems bottomless", "Chasm", "great depth" and "deep hole".)