Mix up the porridge? (4)
I believe the answer is:
stir
I believe this clue is a double definition.
'mix up' is the first definition.
(I've seen this in another clue)
'porridge?' is the second definition.
(both are slang terms for prison)
'the' is the link.
(Other definitions for stir that I've seen before include "move very slightly, or agitate" , "Move; wake" , "Ladies' underwear" , "Uproar" , "Nick" .)