A prison on the move (5)

I believe the answer is:

astir

'on the move' 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 "Up and about -- is art (anag)" , "Busy" , "Excited; out of bed" , "Excitedly moving" , "In a state of movement" .)

I've seen this clue in The Telegraph and The Globe and Mail.

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