The most common and most likely answer to this crossword clue is the 10 letter word CONVENTION
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result CONVENTION
Buddy explains!
The straight part
We know that "Rule normally followed/large formal assembly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rule normally followed/large formal assembly = convention.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 CONVENTION mean?
CONVENTION noun- A meeting or gathering.
- A formal deliberative assembly of mandated delegates.
- The convening of a formal meeting.
- A formal agreement, contract or pact.
- A treaty or supplement to such.
- A practice or procedure widely observed in a group, especially to facilitate social interaction; a custom.