Not still a prison (5)

I believe the answer is:

astir

'not still' is the definition.
(I've seen this in another clue)

'a prison' is the wordplay.
'prison' becomes 'stir' (informal term for prison).
'a'+'stir'='ASTIR'

(Other definitions for astir that I've seen before include "Excitedly moving" , "Busy" , "This isn't still" , "In motion - out of bed" , "Starting to move" .)

I've seen this clue in The Guardian.

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