Knights' quest (5)
I believe the answer is:
grail
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">knights' quest</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)