Merchant possibly tarred? (6)
I believe the answer is:
trader
'merchant' is the definition.
(trader is a kind of merchant)
'possibly tarred?' is the wordplay.
'possibly' indicates an anagram.
'tarred' with letters rearranged gives 'TRADER'.
(Other definitions for trader that I've seen before include "Retard (anag.)" , "One selling" , "Tarred the dealer badly" , "Merchant ship; dealer" , "Dealer, monger" .)