Colourless radioactive element; symbol Rn (5)

I believe the answer is:

radon

(Other definitions for radon that I've seen before include "A radioactive gas, symbol Rn" , "Colourless gas, atomic number 86" , "hazardous substance" , "Element, symbol Rn" , "Rare noble gas" .)

I've seen this clue in The Mirror.

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