Art turning up with the German dealer (6)
I believe the answer is:
trader
'dealer' is the definition.
(I know that dealer is a type of dealer)
'art turning up with the german' is the wordplay.
'turning up' shows that the letters should be reversed in order.
'with' is a charade indicator (letters next to each other).
'the german' becomes 'der' ('the' in German).
'art' written backwards gives 'tra'.
'tra'+'der'='TRADER'
(Other definitions for trader that I've seen before include "Tarred the dealer badly" , "Person in the business of buying and selling" , "One selling" , "Tarred like merchant" , "One in business" .)