The most common and most likely answer to this crossword clue is the 3 letter word JUG
J
U
G
Buddy explains!
The straight part
We know that "Prison" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case prison = jug.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does JUG mean?
JUG noun- A serving vessel or container, typically circular in cross-section and typically higher than it is wide, with a relatively small mouth or spout, an ear handle and often a stopper or top.
- The amount that a jug can hold.
- Jail.
- (chiefly in the plural) A woman's breasts.
- A kettle.
- (chiefly in the plural) A kind of large, high-powered vacuum tube.
- To stew in an earthenware jug etc.
- To put into jail.
- To utter a sound like "jug", as certain birds do, especially the nightingale.
- (of quails or partridges) To nestle or collect together in a covey.
- A small mixed breed of dog created by mating a Jack Russell terrier and a pug.