First person in church is a clergyman (8)
I believe the answer is:
minister
'a clergyman' is the definition.
(minister is a kind of clergyman)
'first person in church' is the wordplay.
'first person' becomes 'i'.
'in' is an insertion indicator.
'church' becomes 'minster' (minster is a kind of church).
'i' going into 'minster' is 'MINISTER'.
'is' is the link.
(Other definitions for minister that I've seen before include "Head of a government department" , "parliamentarian" , "Cabinet member" , "Servant of state or church" , "Member of government or clergy" .)