Trader's French sea song (8)
I believe the answer is:
merchant
'trader's' is the definition.
(thesaurus)
'french sea song' is the wordplay.
'french sea' becomes 'mer' ('sea' in French).
'song' becomes 'chant' (I've seen this before).
'mer'+'chant'='MERCHANT'
(Other definitions for merchant that I've seen before include "Shakespeare: ''The ... of Venice''" , "One who buys and sells" , "that's often associated with speed" , "Wholesale trader" , "One involved in wholesale trade" .)