Formerly part of a population census (4)
I believe the answer is:
once
'formerly' is the definition.
(I know that formerly can be written as once)
'part of a population census' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'ONCE' can be found hidden inside 'population census'.
(Other definitions for once that I've seen before include "In former times" , "One-time - one time" , "Singly" , "in bygone era" , "A single time" .)