The most common and most likely answer to this crossword clue is the 7 letter word CAPTIVE
I've seen this crossword clue in Financial Times
C
A
P
T
I
V
E
Buddy explains!

The straight part
We know that "One in jail" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one in jail = captive.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 CAPTIVE mean?
CAPTIVE noun- One who has been captured or is otherwise confined.
- One held prisoner.
- One charmed or subdued by beauty, excellence, or affection; one who is captivated.
- To capture; to take captive.
- Held prisoner; not free; confined.
- Subdued by love; charmed; captivated.
- Of or relating to bondage or confinement; serving to confine.