Old king, one banning painting, possibly then returning it (10)
I believe the answer is:
artaxerxes
'old king' is the definition.
(I've seen this before)
'one banning painting possibly then returning it' is the wordplay.
'one banning painting possibly' becomes 'art axer' (someone banning or axing art).
'then' is a charade indicator (letters next to each other).
'returning' says the letters should be written in reverse.
'it' becomes 'sex' ('it' is a common euphemism for sex).
'sex' written backwards gives 'xes'.
'artaxer'+'xes'='ARTAXERXES'
(Another definition for artaxerxes that I've seen is " Old king".)