He painted tsar’s opponents, eating only when ordered (8)
I believe the answer is:
reynolds
'he painted' is the definition.
(painter Joshua Reynolds)
'tsar's opponents eating only when ordered' is the wordplay.
'tsar's opponents' becomes 'reds' (the tsar's opponents in the Russian Revolution).
'eating' means one lot of letters goes inside another.
'when ordered' indicates anagramming the letters.
'only' with letters rearranged gives 'ynol'.
'reds' placed around 'ynol' is 'REYNOLDS'.
(Other definitions for reynolds that I've seen before include "Sir Joshua -, English portrait painter" , "Sir Joshua -, 18th century English painter" , "Joshua -, C 18 painter" , "Sir Joshua ---, portraitist" , "Burt -, US actor" .)