Derisive remark about prosperous gem dealer (8)
I believe the answer is:
jeweller
'gem dealer' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
(Other definitions for jeweller that I've seen before include "high-street retailer" , "Maker of bracelets, rings, necklaces, etc" , "High street retailer" , "Seller of rings etc." , "Shopkeeper" .)