At one time part of population census (4)
I believe the answer is:
Here is my best explanation:
'at one time' is the definition.
(I've seen this before)
'part of population census' is the subsidiary indication.
'part of' indicates the answer is hidden within the clue.
'once' is hidden within 'population census'.
(Other definitions for once that I've seen before include "For a single time only", "One time", "In days past", "Previously", "One-time musical film and song", "Single time", "In a former time", "long ago", "in days of yore?" and "not repeatedly".)