When in church for instance (4)
I believe the answer is:
case
'instance' is the definition.
('case' can be a synonym of 'instance')
'when in church' is the wordplay.
'when' becomes 'as'.
'in' means one lot of letters goes inside another.
'church' becomes 'ce' (abbreviation for Church of England**).
'as' going into 'ce' is 'CASE'.
'for' acts as a link.
(Other definitions for case that I've seen before include "See 12" , "proceedings" , "Wine buyer's basic quantity" , "Reconnoitre" , "baggage item" .)