Song from Callas, not her first (4)
I believe the answer is:
aria
'song' is the definition.
(I know that aria is a type of song)
'callas not her first' is the wordplay.
'callas' becomes 'maria' (Maria Callas).
'not her first' means to remove the first letter.
'maria' with its first letter removed is 'ARIA'.
'from' is the link.
(Other definitions for aria that I've seen before include "Song from an opera (4)" , "selection of opera" , "Musical performance" , "Air in an opera" , "Operatic number" .)