Moving article on prison (5)
I believe the answer is:
astir
'moving' is the definition.
(I've seen this before)
'article on prison' is the wordplay.
'article' becomes 'a' (the indefinite article in English).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'prison' becomes 'stir' (informal term for prison).
'a'+'stir'='ASTIR'
(Other definitions for astir that I've seen before include "In motion - out of bed" , "abroad" , "This isn't still" , "Excitedly moving" , "Awake and out of bed" .)