The most common and most likely answer to this crossword clue is the 10 letter word AGGRESSORS
I've seen this crossword clue in The Guardian
A
G
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R
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More clues leading to the result AGGRESSORS
Buddy explains!
The straight part
We know that "they will become belligerent" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they will become belligerent = aggressors.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 AGGRESSORS mean?
AGGRESSORS noun- The person or country that first attacks or makes an aggression; that begins hostility or a quarrel; an assailant.