Performance of Callas, perhaps: missing the start (4)
I believe the answer is:
aria
'performance' is the definition.
(aria is a kind of performance)
'callas perhaps missing the start' is the wordplay.
'callas perhaps' becomes 'maria' (singer Maria Callas).
'missing the start' means to remove the first letter.
'maria' with its first letter taken away is 'ARIA'.
'of' acts as a link.
(Other definitions for aria that I've seen before include "Song from an opera (4)" , "Air in an opera" , "Refrain" , "Repeated strain" , "Operatic solo, duet, etc" .)