Guinevere's lover (8)
I believe the answer is:
lancelot
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">guinevere's lover</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for lancelot that I've seen before include "K?" , "Arthur's knight, lover of Guinevere" , "Most famous of King Arthur's knights, lover of Guinevere" , "Round Table knight who loved Guinevere" , "Knight of the Round Table and close friend of King Arthur's wife, Guinevere" .)