Donkey on a long lead in a splendid meadow (9)
I believe the answer is:
grassland
'meadow' is the definition.
(thesaurus)
'donkey on a long lead in a splendid' is the wordplay.
'donkey' becomes 'ass' (ass is a term for a donkey).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'lead' indicates taking the first letters.
'in' indicates putting letters inside.
'a splendid' becomes 'grand' (both can mean impressive).
The first letter of 'long' is 'l'.
'ass'+'l'='assl'
'assl' placed within 'grand' is 'GRASSLAND'.
(Other definitions for grassland that I've seen before include "(growth area)" , "Large area suitable for grazing" , "green countryside" , "Meadow or prairie" , "Plain" .)