Strauss heroine’s operatic song upset finale (7)
I believe the answer is:
ariadne
'strauss heroine's' is the definition.
(I've seen this in another clue)
'operatic song upset finale' is the wordplay.
'operatic song' becomes 'aria' (I've seen this before).
'upset' is a reversal indicator.
'finale' becomes 'end' (the end of a story or show).
'end' reversed gives 'dne'.
'aria'+'dne'='ARIADNE'
(Other definitions for ariadne that I've seen before include "She aided Theseus against the Minotaur" , "Lover of Theseus" , "beautiful maiden" , "Beautiful female" , "subject of opera" .)