Donkey crossing near deep chasm (5)
I believe the answer is:
abyss
'deep chasm' is the definition.
(I've seen this before)
'donkey crossing near' is the wordplay.
'donkey' becomes 'ass' (ass is a term for a donkey).
'crossing' is an insertion indicator.
'near' becomes 'by' (I've seen this in other clues).
'ass' enclosing 'by' is 'ABYSS'.
(Other definitions for abyss that I've seen before include "Hell; hole" , "Deep gulf" , "something rather deep" , "ocean depth" , "Deep pit" .)