The most common and most likely answer to this crossword clue is the 9 letter word PHALANXES
I've seen this crossword clue in Financial Times
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The straight part
We know that "In assemblies" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in assemblies = phalanxes.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 PHALANXES mean?
PHALANXES noun- (plural phalanxes) An ancient Greek and Macedonian military unit that consisted of several ranks and files (lines) of soldiers in close array with joined shields and long spears.
- (historical sociology) A Fourierite utopian community; a phalanstery.
- (plural phalanxes) A large group of people, animals or things, compact or closely massed, or tightly knit and united in common purpose.
- (plural phalanges) One of the bones of the finger or toe.