The most common and most likely answer to this crossword clue is the 3 plus 7 letter word JAM SESSION
I've seen this crossword clue in Financial Times
J
A
M
S
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S
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More clues leading to the result JAM SESSION
- Improv performance's block seating?
- Crowd sitting for improvised performance
- Join masses dancing for music performance
- Impromptu time for a sarnie?
- Musicians’ gathering blocks discordant noises
- Crowd sitting for impromptu jazz concert
- Playing music will hold-up meeting
- Mobs making noise around front of stage in impromptu musical event
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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.