The most common and most likely answer to this crossword clue is the 12 letter word INSURRECTION
I've seen this crossword clue in The Guardian
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Buddy explains!

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 INSURRECTION mean?
INSURRECTION noun- A violent uprising of part or all of a national population against the government or other authority.