Classical star's note to introduce Mahler's Fourth (3)
I believe the answer is:
sol
'classical star's' is the definition.
('sun' in Latin)
'note to introduce mahler's fourth' is the wordplay.
'note' becomes 'so' (musical note).
'to introduce' is a charade indicator (letters next to each other).
'mahler's fourth' becomes 'l' (4th letter of 'mahler').
'so'+'l'='SOL'
(Other definitions for sol that I've seen before include "Personification of the sun" , "nearby star" , "Note; Peruvian currency" , "The sun in 12 Across" , "Spanish and poetic sun" .)