Dealer of commerce, really (6)
I believe the answer is:
mercer
'dealer' is the definition.
(I know that mercer is a type of dealer)
'of commerce really' is the wordplay.
'of' says the answer is hidden in the clue.
'MERCER' can be found hidden inside 'commerce really'.
(Other definitions for mercer that I've seen before include "Old tradesman" , "Dealer in fine cloth" , "Textile dealer (historical)" , "Dealer in textiles or dry goods" , "Man of the cloth" .)