"William Tell" composer (7)
I believe the answer is:
rossini
'william tell composer' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for rossini that I've seen before include "Composer of 'The Barber of Seville' and 'William Tell'" , "Composer of the opera William Tell, d. 1868" , "whose barber was in Seville" , "he scored regularly" , "Great musician" .)