Society function hosted by European leader for composer (7)
I believe the answer is:
rossini
'composer' is the definition.
(Italian composer Gioachino Rossini)
'society function hosted by european leader' is the wordplay.
'society' becomes 's' (common abbreviation in names of organisations).
'function' becomes 'sin' (mathematical function).
'hosted by' means one lot of letters goes inside another.
'european leader' becomes 'roi' ('king' in French).
's'+'sin'='ssin'
'ssin' going inside 'roi' is 'ROSSINI'.
'for' acts as a link.
(Other definitions for rossini that I've seen before include "Composer of 'The Barber of Seville' and 'William Tell'" , "Largo al factotum composer" , "whose barber was in Seville" , "he produced harmony" , "he scored regularly" .)