Good French painter capturing old man, an emperor’s supporter (11)
I believe the answer is:
bonapartist
'an emperor's supporter' is the definition.
(supporter of Napoleon Bonaparte)
'good french painter capturing old man' is the wordplay.
'good french' becomes 'bon' ('good' in French).
'painter' becomes 'artist' (painter is an example).
'capturing' means one lot of letters goes inside another.
'old man' becomes 'pa' (both can mean father).
'artist' placed around 'pa' is 'apartist'.
'bon'+'apartist'='BONAPARTIST'
(Another definition for bonapartist that I've seen is " Supporter of Napoleon".)