Inmate taken in by amateur brief (7)

I believe the answer is:

laconic

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'brief' is the definition.
(synonyms)

'inmate taken in by amateur' is the wordplay.
'inmate' becomes 'con' (convict).
'taken in by' indicates putting letters inside.
'amateur' becomes 'laic' (I can't explain this - if you can you should give a lot more credence to this answer).
'con' going inside 'laic' is 'LACONIC'.

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(Other definitions for laconic that I've seen before include "Saying little" , "In short" , "Concise - terse" , "'Curt, terse (7)'" , "Terse; pithy" .)

I've seen this clue in The Telegraph.

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