Guinevere's lover (8)
I believe the answer is:
lancelot
'guinevere's lover' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)