Number turned to the Church, in the past (4)
I believe the answer is:
once
'in the past' is the definition.
(I've seen this before)
'number turned to the church' is the wordplay.
'number' becomes 'no' (abbreviation for 'number'**).
'turned' is a reversal indicator.
'to the church' becomes 'ce' (abbreviation for Church of England).
'no' reversed gives 'on'.
'on'+'ce'='ONCE'
(Other definitions for once that I've seen before include "in days of yore?" , "yesterday" , "'... bitten, twice shy'" , "On a single (or past) occasion" , "In former times" .)