The most common and most likely answer to this crossword clue is the 9 letter word IMPOUNDED
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More clues leading to the result IMPOUNDED
Buddy explains!

The straight part
We know that "locked up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case locked up = impounded.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 IMPOUNDED mean?
IMPOUNDED verb- To shut up or place in an enclosure called a pound
- To hold back (for example water by a dam)
- To hold in the custody of a court or its delegate
- To collect and hold (funds) for payment of property taxes and insurance on property in which one has a security interest