Radioactive gas (5)

I believe the answer is:

radon

'radioactive gas' is the definition.
(I've seen this before)

This is the entire clue.

(Other definitions for radon that I've seen before include "Element, symbol Rn" , "Inert gas that occurs naturally" , "A radioactive gas, symbol Rn" , "Natural gas considered a hazard to health" , "Gaseous element, Rn" .)

I've seen this clue in the USA Today, The Telegraph, The Times and The Guardian.

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