The most common and most likely answer to this crossword clue is the 5 plus 4 letter word SNARE DRUM
I've seen this crossword clue in The Guardian
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More clues leading to the result SNARE DRUM
- Producer of rolls finally invests in a famous racehorse
- Butcher and a pair of Poles sent up one given stick
- A racehorse behind the leaders in sixth, naturally one's beaten
- Seized strange instrument
- Trapped by strange instrument
- Instrument reading as if in Arabic, rub out answer?
- The recipient of regular beating is to crack, shortly, returning to kill
- Killing man’s heading off back — beat it!
- Caught spirit with percussion instrument
- South American admitting new crime over instrument
Buddy explains!

The straight part
We know that "beat it" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case beat it = snare drum.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.