Holy chalice of legend (5)
I believe the answer is:
'holy chalice of legend' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for grail that I've seen before include "Cherished goal" , "Arthurian knights' quest" , "Holy object of search" , "Object of King Arthur's quest" , "sought by Parsifal" .)