King Arthur's island (6)
I believe the answer is:
avalon
'king arthur's island' 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 "Isle of the Blessed in Arthurian legend" , "King Arthur's burial place" , "Where King Arthur's body was brought" , "Mythical isle" , "Isle of Celtic myth" .)