Radioactive element and/or manufactured gas (5)

I believe the answer is:

radon

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'radioactive element' is the definition.
(I've seen this in another clue)

'andor manufactured gas' is the wordplay.
'andor' can be anagrammed to 'RADON'.
However, I'm not clear how this is indicated.

Can you help me to learn more?

(Other definitions for radon that I've seen before include "A radioactive gas, symbol Rn" , "Naturally occurring inert gas" , "gets no reaction" , "Inert gas - adorn (anag)" , "Gaseous element, Rn" .)

I've seen this clue in The Guardian.

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