A priest is one? (8)
I believe the answer is:
cardinal
I believe this clue is a double definition.
'a priest' is the first definition.
(I know that cardinal is a type of bishop)
'is one?' is the second definition.
(similar in meaning)
(Other definitions for cardinal that I've seen before include "of prime importance" , "Counsellor to the Pope" , "Key" , "Senior churchman - bird" , "Type of number or prelate" .)