A priest - one in a church (8)
I believe the answer is:
minister
'a priest' is the definition.
(I've seen this before)
'one in a church' is the wordplay.
'one' becomes 'I' (Roman numeral).
'in' is an insertion indicator.
'a church' becomes 'minster' (minster is a kind of church).
'i' placed within 'minster' is 'MINISTER'.
(Other definitions for minister that I've seen before include "Member of parliament" , "Member of government or clergy" , "Political or religious official" , "supply as before" , "Tend" .)