Fabled holy vessel (5)
I believe the answer is:
grail
'fabled holy vessel' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for grail that I've seen before include "Legendary cup sought by knights" , "Chalice used by Jesus at the Last Supper" , "Round Table Knights sought the Holy ....." , "Holy vessel used at the Last Supper" , "Famous chalice" .)