Arthurian knight, Guinevere's lover (8)
I believe the answer is:
lancelot
I believe this is a double definition.
'arthurian knight' is the first definition.
(knight in Arthurian legends)
'guinevere's lover' is the second definition.
(I've seen this in another clue)
(Other definitions for lancelot that I've seen before include "Knight of the Round Table and close friend of King Arthur's wife, Guinevere" , "K?" , "Call tone for knight of Arthurian legend" , "A Knight of the Round Table, lover of Guinevere (8)." , "Round Table member" .)