Hazardous gas (5)

I believe the answer is:

radon

'hazardous gas' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for radon that I've seen before include "A radioactive gas, symbol Rn" , "Radioactive gas, linked to granite" , "Noble gas, Rn" , "Rare noble gas" , "Naturally occurring inert gas" .)

I've seen this clue in the Penny Press, the Newsday and the LA Times.

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