Crossword Genius

Leave military formation in radioactive dust cloud (7)

Ross

I believe the answer is:

fallout

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'dust cloud' is the definition.
(thesaurus)

I don't understand how the rest of the clue works.

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(Other definitions for fallout that I've seen before include "Quarrel - leave the ranks" , "Side-effects" , "Adverse aftermath" , "Quarrel, or leave place in military formation" , "after-effect" .)

I've seen this clue in The Telegraph.
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