Tristram's beloved (6)
I believe the answer is:
isolde
'tristram's beloved' 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 "King Mark's betrothed (Wagner)" , "Operatic heroine" , "Irish princess" , "love potion drinker" , "Soiled like Tristram's medieval lover" .)