Dealer putting up left-wing paintings? (6)
I believe the answer is:
trader
'dealer' is the definition.
(I know that dealer can be written as trader)
'up left-wing paintings?' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'left wing' becomes 'red' (red can informally mean communist).
'paintings?' becomes 'art' (painting is a type of artwork).
'red'+'art'='redart'
'redart' back-to-front is 'TRADER'.
'putting' acts as a link.
(Other definitions for trader that I've seen before include "Tarred the dealer badly" , "He buys and sells goods" , "One in business" , "One selling" , "Merchant ship; dealer" .)