Donkey crossing near crevasse (5)
I believe the answer is:
abyss
'crevasse' is the definition.
(thesaurus)
'donkey crossing near' is the wordplay.
'donkey' becomes 'ass' (I've seen this before).
'crossing' means one lot of letters goes inside another.
'near' becomes 'by' (I've seen this in other clues).
'ass' placed around 'by' is 'ABYSS'.
(Other definitions for abyss that I've seen before include "Fathomless pit" , "Deep gulf" , "Hell; hole" , "Deep hole, seems bottomless" , "Very large and deep chasm" .)