I believe the answer is:
'wagner heroine' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for isolde that I've seen before include "Tristan and . . . . . . (medieval romance)" , "Irish princess" , "Tristram's lover, quite soiled" , "Soiled like Tristan's lover" , "Soiled like Tristram's girl friend" .)