Callas misses first part of opera (4)
I believe the answer is:
aria
'part of opera' is the definition.
('aria' is part of 'opera')
'callas misses first' is the wordplay.
'callas misses' becomes 'ari' (I can't explain this - if you can you should believe this answer much more).
'first' becomes 'a' (first letter of the alphabet).
'ari'+'a'='ARIA'
(Other definitions for aria that I've seen before include "Vocal solo in a cantata etc" , "Operatic number" , "Repeated strain" , "Song, one from 'West Side Story' needing no introduction" , "Imitative art work" .)