Tristram's beloved (6)
I believe the answer is:
isolde
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">tristram's beloved</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)