French sea song from a trader (8)
I believe the answer is:
merchant
'a trader' is the definition.
(I know that trader can be written as merchant)
'french sea song' is the wordplay.
'french sea' becomes 'mer' ('sea' in French).
'song' becomes 'chant' (song sung by a crowd).
'mer'+'chant'='MERCHANT'
'from' acts as a link.
(Other definitions for merchant that I've seen before include "Businessperson engaged in trade" , "Retail trader" , "Antonio, say" , "Ten march (anag.)" , "Distributor" .)