The most common and most likely answer to this crossword clue is the 4 plus 6 letter word OPEN PRISON
I've seen this crossword clue in The Daily Telegraph
O
P
E
N
P
R
I
S
O
N
More clues leading to the result OPEN PRISON
- Cooler, more easy-going
- Write page in prayer where one has a little freedom?
- Nick the honest priest is working
- Work composed in person, wherein sentences aren’t rigidly controlled
- Love quiet reformed person in penal establishment
- In which a stretch is more comfortable?
- Where those captive are free?
- Penitentiary with unlimited accommodation
Buddy explains!

The straight part
We know that "penal establishment" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case penal establishment = open prison.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.