Revolutionary painting put up by the dealer (6)
I believe the answer is:
trader
'the dealer' is the definition.
('trader' can be a synonym of 'dealer')
'revolutionary painting put up' is the wordplay.
'revolutionary' becomes 'red' (red can informally mean communist).
'painting' becomes 'art' (painting is an art).
'put up' shows that the letters should be reversed in order.
'red'+'art'='redart'
'redart' backwards is 'TRADER'.
'by' is the link.
(Other definitions for trader that I've seen before include "Dealer, monger" , "Tarred the dealer badly" , "Person in the business of buying and selling" , "He buys and sells goods" , "Tarred like merchant" .)