Formerly found in a rehabilitation centre (4)
I believe the answer is:
once
'formerly' is the definition.
(I know that formerly can be written as once)
'in a rehabilitation centre' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ONCE' is hidden in the letters of 'rehabilitation centre'.
'found' acts as a link.
(Other definitions for once that I've seen before include "Long ago" , "In days gone by" , "in bygone era" , "not repeatedly" , "At some time past" .)