Fool around next to a deep hole (5)
I believe the answer is:
abyss
'a deep hole' is the definition.
(I've seen this in another clue)
'fool around next to' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'around' indicates putting letters inside.
'next to' becomes 'by'.
'ass' going around 'by' is 'ABYSS'.
(Other definitions for abyss that I've seen before include "deep depression" , "Deep pit" , "Deep hole, seems bottomless" , "something rather deep" , "Seemingly bottomless chasm" .)