Ancient study with group of horses he painted (9)
I believe the answer is:
constable
'he painted' is the definition.
(painter John Constable)
'ancient study with group of horses' is the wordplay.
'ancient study' becomes 'con' (old-fashioned word meaning to study).
'with' is a charade indicator (letters next to each other).
'group of horses' becomes 'stable'.
'con'+'stable'='CONSTABLE'
(Other definitions for constable that I've seen before include "Copper - artist" , "CARRY ON FILM" , "the governor of a royal castle" , "In England, a great painter or lowly policeman" , "Bobby" .)