A prison in motion (5)
I believe the answer is:
astir
'in motion' 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 "Starting to move" , "In a state of movement" , "In motion - out of bed" , "in the land of the living" , "Out of bed or on the move" .)