Having old prophet on staff sounds like quite a trial (8)
I believe the answer is:
crucible
'trial' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for crucible that I've seen before include "Miller's bewitching play" , "Melting-pot" , "smelting vessel" , "Pot in which metals are melted" , "Melting pot" .)