Donkey left in very good pasture (9)
I believe the answer is:
grassland
'pasture' is the definition.
(I've seen this before)
'donkey left in very good' is the wordplay.
'donkey' becomes 'ass' (ass is a term for a donkey).
'left' becomes 'l' (common abbreviation).
'in' is an insertion indicator.
'very good' becomes 'grand' (I've seen this in other clues).
'ass'+'l'='assl'
'assl' placed inside 'grand' is 'GRASSLAND'.
(Other definitions for grassland that I've seen before include "(growth area)" , "Plain" , "green countryside" , "pasture" , "Large area suitable for grazing" .)