Leave military formation in radioactive dust cloud (7)
I believe the answer is:
fallout
'dust cloud' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(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" .)