Diva ____ Callas (5)
I believe the answer is:
maria
'diva callas' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for maria that I've seen before include "Dark planes on the moon; girl's name" , "More than one plain" , "Heroine of West Side Story" , "Black --, police van" , "Italian Mary" .)