Knights' quest (5)
I believe the answer is:
grail
'knights' quest' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for grail that I've seen before include "Last Supper cup" , "Holy vessel used at the Last Supper" , "Arthurian knights' quest" , "Object of King Arthur's quest" , "sought by Parsifal" .)