Pit donkey that's kept by (5)
I believe the answer is:
abyss
'pit' is the definition.
(I've seen this in another clue)
'donkey that's kept by' is the wordplay.
'donkey' becomes 'ass' (ass is a term for a donkey).
'that's kept' is an insertion indicator.
'ass' placed around 'by' is 'ABYSS'.
(Other definitions for abyss that I've seen before include "Very deep gorge" , "Measureless chasm" , "crevasse" , "great depth" , "Deep hole, seems bottomless" .)