Princess in a Wagner opera
I believe the answer is:
isolde
'princess in a wagner opera' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for isolde that I've seen before include "King Mark's betrothed (Wagner)" , "Tristan and . . . . . . (medieval romance)" , "princess of legend" , "Operatic heroine" , "Wagnerian heroine" .)