On the church in days gone by (4)
I believe the answer is:
once
'in days gone by' is the definition.
(I've seen this before)
'on the church' is the wordplay.
'the church' becomes 'ce' (abbreviation for Church of England).
'on'+'ce'='ONCE'
(Other definitions for once that I've seen before include "Formerly; as soon as" , "Formerly - not repeated" , "In days past" , "In times past" , "in bygone days" .)