Beast of burden going round by a deep gorge (5)
I believe the answer is:
abyss
'a deep gorge' is the definition.
(I've seen this in another clue)
'beast of burden going round by' is the wordplay.
'beast of burden' becomes 'ass' (animal which carries a load).
'going round' indicates putting letters inside (some letters go around others).
'ass' going around 'by' is 'ABYSS'.
(Other definitions for abyss that I've seen before include "Deep pit" , "great depth" , "Deep gulf" , "Seemingly bottomless chasm" , "Fathomless pit" .)