Dealer put up Communist paintings (6)
I believe the answer is:
trader
'dealer' is the definition.
(I know that dealer can be written as trader)
'up communist paintings' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'communist' becomes 'red' (red can informally mean communist).
'paintings' becomes 'art' (painting is an art).
'red'+'art'='redart'
'redart' written backwards gives 'TRADER'.
'put' acts as a link.
(Other definitions for trader that I've seen before include "Retard (anag.)" , "Tarred like merchant" , "Person in the business of buying and selling" , "He buys and sells goods" , "One in business" .)