Mix in a jug (4)
I believe the answer is:
stir
I believe this is a double definition.
'mix in' is the first definition.
(I've seen this in another clue)
'a jug' is the second definition.
(both are slang for prison)
(Other definitions for stir that I've seen before include "Big house" , "Budge" , "Shake up" , "jug" , "Thrill" .)