One in church? (8)
I believe the answer is:
minister
'one in church?' is the definition.
'one in church?' is the wordplay.
'one' becomes 'I' (Roman numeral).
'in' indicates putting letters inside.
'church?' becomes 'minster' (minster is a kind of church).
'i' put into 'minster' is 'MINISTER'.
(Other definitions for minister that I've seen before include "Member of government or clergy" , "Head of a government department" , "Parson" , "Holder of office in government or church" , "Tend" .)