Instance of arsenic being brought into church (4)
I believe the answer is:
case
'instance' is the definition.
(synonyms)
'arsenic being brought into church' is the wordplay.
'arsenic' becomes 'As' (As is the chemical symbol for arsenic).
'being brought into' indicates putting letters inside.
'church' becomes 'ce' (abbreviation for Church of England).
'as' placed inside 'ce' is 'CASE'.
'of' is the link.
(Other definitions for case that I've seen before include "Box or outer covering" , "Patient; bag" , "Instance - container" , "Wine buyer's quantity - dative, for example" , "Reconnoitre" .)