The most common and most likely answer to this crossword clue is the 4 plus 5 letter word SING ALONG
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Buddy explains!
The straight part
We know that "Join a chorus" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case join a chorus = sing along.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 SING ALONG mean?
SING ALONG noun- A gathering or event where participants are encouraged to add their voices in song.