Mr N is in church (7)
I believe the answer is:
minster
'church' is the definition.
(I know that minster is a type of cathedral)
'mr n is in' is the wordplay.
'mr' becomes 'mister' (Mr is short for Mister).
'is in' is an insertion indicator.
'mister' placed around 'n' is 'MINSTER'.
(Other definitions for minster that I've seen before include "Large church (eg, York)" , "Large church (or cathedral at York)" , "Old monastic cathedral, such as York's" , "Abbey or priory church" , "Large important church" .)