The most common and most likely answer to this crossword clue is the 3 plus 6 letter word ILL GOTTEN
I've seen this crossword clue in Financial Times
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More clues leading to the result ILL GOTTEN
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The straight part
We know that "Knocked off" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case knocked off = ill gotten.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 ILL GOTTEN mean?
ILL GOTTEN adjective- Obtained improperly or illegally.