Composer by trade, you say? (7)
I believe the answer is:
purcell
'composer' is the definition.
(Henry Purcell 1659-1696)
'by trade you say?' is the wordplay.
'by' becomes 'pur' (I can't justify this - if you can you should give a lot more credence to this answer).
'trade' becomes 'sell' (I've seen this in another clue).
'you say?' indicates a 'sounds like' (homophone) clue.
'sell' sounds like 'cell'.
'pur'+'cell'='PURCELL'
Can you help me to learn more?
(Other definitions for purcell that I've seen before include "Composer of The Fairy Queen, d.1695" , "organist" , "Henry --, English baroque composer" , "English composer (Dido and Aeneas)" , "Composer of Dido and Aeneas, d. 1695" .)