Previously held in detention centre (4)

I believe the answer is:

once

'previously' is the definition.
(once means in the past)

'held in detention centre' is the wordplay.
'held in' says the answer is hidden in the clue.
'ONCE' is hidden in the letters of 'detention centre'.

(Other definitions for once that I've seen before include "A single time" , "Some time back" , "Singly" , "in bygone days" , "Formerly, at one time" .)

I've seen this clue in The Independent, The Sun and The Times.

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