Colourless, odourless inert gas, Ar, used in some electric lights (5)

I believe the answer is:

argon

(Other definitions for argon that I've seen before include "Inert eighteen ac gas" , "something inert and insubstantial" , "Gas, 1% of atmosphere" , "Inert gas, symbol Ar" , "Gas used in electric lights" .)

I've seen this clue in The Guardian.

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