Previously seen in vaccination centre (4)
I believe the answer is:
once
'previously' is the definition.
(I've seen this in another clue)
'in vaccination centre' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ONCE' is hidden in the letters of 'vaccination centre'.
'seen' acts as a link.
(Other definitions for once that I've seen before include "On a single (or past) occasion" , "In the old days" , "In times past" , "At some time past" , "in bygone era" .)