Chalice used by Christ at the Last Supper (4,5)
I believe the answer is:
holy grail
'chalice used by christ at the last supper' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for holy grail that I've seen before include "Cup that was much sought after?" , "Something eagerly sought" , "Christ's cup at the Last Supper, sought by medieval knights" , "Legendary cup used by Jesus at Last Supper" , "In legend, the cup used by Christ at the Last Supper" .)