They trade in French sea songs (9)
I believe the answer is:
merchants
'they trade' is the definition.
Although both the answer and 'trade' are nouns, they are not in the same form of the noun.
'french sea songs' is the wordplay.
'french sea' becomes 'mer' ('sea' in French).
'songs' becomes 'chants' (I've seen this before).
'mer'+'chants'='MERCHANTS'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for merchants that I've seen before include "Traders in large quantities" , "They buy and sell" , "Business people engaged in trade" , "Retail traders" .)