Composer admits second sin to French king (7)
I believe the answer is:
rossini
'composer' is the definition.
(I've seen this before)
'admits second sin to french king' is the wordplay.
'admits' means one lot of letters goes inside another.
'second' becomes 's' (common abbreviation).
'to french king' becomes 'roi' (I've seen this before).
's'+'sin'='ssin'
'ssin' inserted inside 'roi' is 'ROSSINI'.
(Other definitions for rossini that I've seen before include "Italian scorer" , "whose barber was in Seville" , "he produced harmony" , "Composer of 'The Barber of Seville' and 'William Tell'" , "Italian composer, 1792-1868" .)