Communist subject turned marketeer (6)
I believe the answer is:
trader
'marketeer' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see an association between them that I don't see?
'communist subject turned' is the wordplay.
'communist' becomes 'red' (red is a kind of communist).
'subject' becomes 'art' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'turned' is a reversal indicator.
'red'+'art'='redart'
'redart' backwards is 'TRADER'.
Can you help me to learn more?
(Other definitions for trader that I've seen before include "Tarred the dealer badly" , "Red tar (anag)" , "Retard (anag.)" , "Tarred like merchant" , "One selling" .)