Previously included in London census (4)
I believe the answer is:
once
'previously' is the definition.
(I've seen this before)
'included in london census' is the wordplay.
'included in' says the answer is hidden in the clue.
'ONCE' is hidden within 'london census'.
(Other definitions for once that I've seen before include "in days of yore?" , "in bygone days" , "Sometime" , "In days gone by" , "Long ago" .)