Composer admitting small sin to French king (7)
I believe the answer is:
rossini
'composer' is the definition.
(I've seen this before)
'admitting small sin to french king' is the wordplay.
'admitting' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'to french king' becomes 'roi' (I've seen this before).
's'+'sin'='ssin'
'ssin' going inside 'roi' is 'ROSSINI'.
(Other definitions for rossini that I've seen before include "Staff writer" , "Composer of 'The Barber of Seville' and 'William Tell'" , "Italian scorer" , "Composer of The Thieving Magpie" , "Italian opera composer" .)