Exciting group supporting prison (8)
I believe the answer is:
stirring
'exciting' is the definition.
('stir' can be a synonym of 'excite')
'group supporting prison' is the wordplay.
'group' becomes 'ring'.
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'prison' becomes 'stir' (informal term for prison).
'ring' put after 'stir' is 'STIRRING'.
(Other definitions for stirring that I've seen before include "Inspiring" , "Setting liquid in motion" , "Rousing; mixing" , "Agitating a liquid" , "Heady" .)