Prison group agitating (8)
I believe the answer is:
stirring
'agitating' is the definition.
(stirring is a kind of agitating)
'prison group' is the wordplay.
'prison' becomes 'stir' (informal term for prison).
'group' becomes 'ring'.
'stir'+'ring'='STIRRING'
(Other definitions for stirring that I've seen before include "Rousing; mixing" , "Rousing (using a spoon?)" , "Heady" , "Setting liquid in motion" , "Exciting" .)