Island of Arthurian legend (6)
I believe the answer is:
avalon
'island of arthurian legend' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for avalon that I've seen before include "Mythical isle" , "Morgan le Fay's isle" , "Arthur's Seat" , "Where King Arthur was conveyed after death" , "Isle of Celtic myth" .)