Start to wake up in prison (4)
I believe the answer is:
stir
I believe this is a double definition.
'start to wake up' is the first definition.
(stir can mean to begin to wake from sleep)
'prison' is the second definition.
(informal term for prison)
'in' acts as a link.
(Other definitions for stir that I've seen before include "Affect emotionally" , "Whisk, blend" , "Excitement" , "Whisk or make a move" , "Whisk or rouse" .)