Fool sheltering next to deep chasm (5)
I believe the answer is:
abyss
'deep chasm' is the definition.
(I've seen this in another clue)
'fool sheltering next to' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'sheltering' means one lot of letters goes inside another.
'next to' becomes 'by'.
'ass' enclosing 'by' is 'ABYSS'.
(Other definitions for abyss that I've seen before include "Bottomless pit" , "Very large and deep chasm" , "Deep pit" , "Measureless chasm" , "depths of sea" .)