At one time part of population census (4)
I believe the answer is:
once
'at one time' is the definition.
(thesaurus)
'part of population census' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'ONCE' is hidden in the letters of 'population census'.
(Other definitions for once that I've seen before include "Quick examination" , "in days of yore?" , "not repeatedly" , "In times past" , "Formerly - not repeated" .)