The most common and most likely answer to this crossword clue is the 9 letter word COMMITTAL
I've seen this crossword clue in The Times
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The straight part
We know that "imprisonment" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case imprisonment = committal.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 COMMITTAL mean?
COMMITTAL noun- The act of entrusting something to someone.
- The act of committing someone to confinement; an order for someone's imprisonment.
- The act of perpetrating an offence.
- The act of committing a body to the grave at a burial or to the furnace at a cremation.
- Of or relating to a committee.
- Of or relating to commitment.