Priest tells senior clergy (8)
I believe the answer is:
Here is my best explanation:
'senior clergy' is the definition.
(I've seen this before)
'priest tells' is the subsidiary indication.
'priest' becomes 'p' (ecclesiastical abbreviation).
'tells' becomes 'relates' (relating is a kind of telling).
(Other definitions for prelates that I've seen before include "Senior clergy", "Senior church dignitories" and "High-ranking clergymen".)