In France I joined Sam, a dealer in diamonds (8)
I believe the answer is:
jeweller
'a dealer in diamonds' is the definition.
'in france i joined sam' is the wordplay.
'in france i' becomes 'je' ('I' in French).
'joined' means one lot of letters go next to another.
'sam' becomes 'weller' (Charles Dickens character Sam Weller).
'je'+'weller'='JEWELLER'
(Other definitions for jeweller that I've seen before include "Seller of gems and precious metals" , "Retailer in gemstone products" , "Seller of rings etc." , "Seller of e.g. rings" , "Trader" .)