The most common and most likely answer to this crossword clue is the 10 letter word PANOPTICON
I've seen this crossword clue in The Times
P
A
N
O
P
T
I
C
O
N
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 = panopticon.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 PANOPTICON mean?
PANOPTICON noun- A type of prison where all the cells are visible from the center, particularly if it is not possible for someone in a cell to know if they are being watched.
- A room for the exhibition of novelties.