Part of opera Callas initially rejected (4)
I believe the answer is:
aria
'part of opera' is the definition.
('aria' is part of 'opera')
'callas initially rejected' is the wordplay.
'callas' becomes 'maria' (Maria Callas).
'initially rejected' means to remove the first letter.
'maria' with its first letter taken off is 'ARIA'.
(Other definitions for aria that I've seen before include "Imitative art work" , "Operatic air" , "Refrain" , "Song, one from 'West Side Story' needing no introduction" , "Piece of music" .)