He is one in church! (8)
I believe the answer is:
minister
'he' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'one in church' is the wordplay.
'one' becomes 'I' (Roman numeral).
'in' means one lot of letters goes inside another.
'church' becomes 'minster' (minster is a kind of church).
'i' placed within 'minster' is 'MINISTER'.
'is' is the link.
Can you help me to learn more?
(Other definitions for minister that I've seen before include "Merit sin (anag.)" , "Senior politician" , "Attend to priest" , "Clergyman - government member" , "Holder of office in government or church" .)