Official one in church (8)
I believe the answer is:
minister
'official' is the definition.
(I know that minister is a type of minister)
'one in church' is the wordplay.
'one' becomes 'I' (Roman numeral).
'in' is an insertion indicator.
'church' becomes 'minster' (minster is a kind of church).
'i' put inside 'minster' is 'MINISTER'.
(Other definitions for minister that I've seen before include "Member of the clergy or of the Front Bench" , "Agent" , "Head of a government department" , "Servant of state or church" , "supply as before" .)