Flemish old master and American country bumpkin, polar opposites (6)
I believe the answer is:
rubens
'flemish old master and american country' is the definition.
I can't judge whether this definition defines the answer.
'bumpkin polar opposites' is the wordplay.
'bumpkin' becomes 'rube' (synonyms).
'polar opposites' becomes 'ns' (I can't justify this - if you can you should believe this answer much more).
'rube'+'ns'='RUBENS'
Can you help me to learn more?
(Other definitions for rubens that I've seen before include "Peter Paul -, sixteen to seventeenth century Flemish painter" , "Peter Paul -, sixteenth to seventeenth century Flemish painter" , "C16-17 Flemish painter" , "Sir Peter Paul ---, painter" , "Flemish baroque artist" .)