Donkey shelters beside a deep gorge (5)
I believe the answer is:
abyss
'a deep gorge' is the definition.
(I've seen this in another clue)
'donkey shelters beside' is the wordplay.
'donkey' becomes 'ass' (ass is a term for a donkey).
'shelters' is an insertion indicator.
'beside' becomes 'by'.
'ass' enclosing 'by' is 'ABYSS'.
(Other definitions for abyss that I've seen before include "depths of sea" , "canyon?" , "gaping hole" , "Very large and deep chasm" , "Deep pit" .)