Crossword Genius

Starts to notice habitual halitosis, having, in elemental form, gas (7)

Ross

I believe the answer is:

ammonia

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'gas' is the definition.
(I know that ammonia is a gas)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for ammonia that I've seen before include "Gas released by smelling salts" , "Colourless gas with strong smell" , "compound?" , "Gas; cleaning fluid" , "Pungent cleaning fluid" .)

I've seen this clue in The Independent.
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