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Some time ago confined in detention centre (4)

Ross

I believe the answer is:

once

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'some time ago' is the definition.
(thesaurus)

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

'confined' acts as a link.
This may not be right. It may be part of another bit of the clue.

Can you help me to learn more?

(Other definitions for once that I've seen before include "Some time back" , "One-time musical film and song" , "In times past" , "A single time" , "Cone (anag.)" .)

I've seen this clue in The Irish Times and the Irish Times (Crosaire).
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