Wagner heroine (6)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">wagner heroine</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)