Opening remark about church people (12)
I believe the answer is:
commencement
'opening' is the definition.
('commencement' can be a synonym of 'opening')
'remark about church people' is the wordplay.
'remark' becomes 'comment' (commenting is a kind of remarking**).
'about' is an insertion indicator.
'church' becomes 'ce' (abbreviation for Church of England).
'people' becomes 'men' (man is a kind of person).
'ce'+'men'='cemen'
'comment' enclosing 'cemen' is 'COMMENCEMENT'.
(Other definitions for commencement that I've seen before include "Origin" , "Beginning" , "Start" .)