Special bit of naughtiness concealed by French king’s composer (7)
I believe the answer is:
rossini
'composer' is the definition.
(Italian composer Gioachino Rossini)
'special bit of naughtiness concealed by french king's' is the wordplay.
'special' becomes 'S' (common abbreviation as in Special Air Service).
'bit of naughtiness' becomes 'sin' (I can't justify this - if you can you should believe this answer much more).
'concealed by' means one lot of letters goes inside another.
'french king' becomes 'roi' ('king' in French).
's'+'sin'='ssin'
'ssin' placed inside 'roi' is 'ROSSINI'.
Can you help me to learn more?
(Other definitions for rossini that I've seen before include "Staff writer" , "Italian composer, 1792-1868" , "Barber of Seville writer" , "Composer of the opera William Tell, d. 1868" , "he produced harmony" .)