Falsetto's cast in part in opera (5)
I believe the answer is:
tosca
'opera' is the definition.
(Giacomo Puccini opera)
'falsetto's cast in part in' is the wordplay.
'in' indicates a hidden word.
'TOSCA' is hidden in the letters of 'falsettos cast part'.
(Other definitions for tosca that I've seen before include "Opera - a Scot (anag)" , "Luigi -, Italian playwright" , "Coats off for Pucccni's opera" , "Opera at Ascot?" , "celebrated singer" .)