The most common and most likely answer to this crossword clue is the 5 letter word CAGED
I've seen this crossword clue in The Guardian
C
A
G
E
D
More clues leading to the result CAGED
- Shut up, being a century old
- Penned record covering a number of years
- Composer died behind bars
- Cold in prison
- Charlie grew old behind bars
- Locked up and cold — what 21 Down may be
- Imprisoned, Conservative put on years
- Locked up composer with daughter
- Penned most of page in post-7 medium
- Caught pensioners in prison
Buddy explains!

The straight part
We know that "in prison" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in prison = caged.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 CAGED mean?
CAGED verb- To confine in a cage; to put into and keep in a cage.
- To restrict someone's movement or creativity.
- To track individual responses to direct mail, either to maintain and develop mailing lists or to identify people who are not eligible to vote because they do not reside at the registered addresses.
- Confined in a cage.
- Of eggs: produced by birds confined in cages; not free-range.