Formerly part of detention centre (4)
I believe the answer is:
once
'formerly' is the definition.
(I know that formerly can be written as once)
'part of detention centre' is the wordplay.
'part of' indicates a hidden word.
'ONCE' is hidden within 'detention centre'.
(Other definitions for once that I've seen before include "On a single (or past) occasion" , "in days of yore?" , "in bygone days" , "Singly" , "In former times" .)