Mix porridge (4)
I believe the answer is:
stir
This clue is a double definition.
'mix' is the first definition.
(I've seen this before)
'porridge' is the second definition.
(both are slang terms for prison)
(Other definitions for stir that I've seen before include "Thrill" , "Uproar" , "Start to move" , "Awaken" , "Wake; disturbance" .)