Ground of the Arthurian legend (5)
I believe the answer is:
earth
'ground' is the definition.
(synonyms)
'of the arthurian legend' is the wordplay.
'of' indicates the answer is hidden within the clue.
'EARTH' can be found hidden inside 'the arthurian legend'.
(Other definitions for earth that I've seen before include "Our favourite planet" , "World - soil" , "Third planet from the Sun" , "Ground - globe - soil" , "Fox's burrow" .)