Gas, O (6)

I believe the answer is:

oxygen

I believe this is a double definition.

'gas' is the first definition.
(I know that oxygen is a type of gas)

'o' is the second definition.
(O is the chemical symbol for oxygen)

(Other definitions for oxygen that I've seen before include "Atmospheric gas" , "Life supporting element" , "Second commonest element in air" , "Gas forming about 20 per cent of atmosphere" , "Most abundant element in the earth's crust" .)

I've seen this clue in The Guardian.

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