Limitlessly rich ore that filled ancient vessels (5)
I believe the answer is:
ichor
'that filled ancient vessels' is the definition.
(bodily fluid of the Greek gods)
'limitlessly rich ore' is the wordplay.
'ICHOR' is hidden within the letters.
But, I am uncertain how this is indicated.
(Other definitions for ichor that I've seen before include "Choir (anag.); gods' life force" , "Juice of the veins of the gods" , "divine stuff" , "bloodlike fluid?" , "Divine liquid" .)