The most common and most likely answer to this crossword clue is the 11 letter word CONVOCATION
I've seen this crossword clue in The Times
C
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Buddy explains!
The straight part
We know that "Assembly of folk brought together by crime, say" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case assembly of folk brought together by crime, say = convocation.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 CONVOCATION mean?
CONVOCATION noun- The act of calling or assembling by summons.
- An assembly or meeting.
- An assembly of the clergy, by their representatives, to consult on ecclesiastical affairs.
- An academic assembly, in which the business of the university is transacted.