Mix in prison (4)
I believe the answer is:
stir
This clue is a double definition.
'mix in' is the first definition.
(I've seen this in another clue)
'prison' is the second definition.
(informal term for prison)
(Other definitions for stir that I've seen before include "Whisk, agitate" , "Prison (slang) - disturbance" , "Move; wake" , "Ladies' underwear" , "Commotion - prison" .)