Tristan's lover (6)
I believe the answer is:
isolde
'tristan's lover' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for isolde that I've seen before include "Arthurian queen" , "Wagnerian heroine" , "King Mark's betrothed (Wagner)" , "love potion drinker" , "heroine (sic)" .)