The most common and most likely answer to this crossword clue is the 9 letter word LARCENIST
I've seen this crossword clue in Independent
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More clues leading to the result LARCENIST
- Petty thief left with dangerous criminal, we hear
- Person stealing new clarinets
- Petty thief in Lima introducing fire raiser on radio?
- Criminal manoeuvres in cartels
- One who takes lives in central development
- One knocks off arrangement for clarinets
- Thief initially lambasts another criminal on radio
- Liberal listened to criminal and thief
- Criminal playing second clarinet
- Criminal certain to nick shilling after pound?
Buddy explains!

The straight part
We know that "One knocks off" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one knocks off = larcenist.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.