Starts to notice habitual halitosis, having, in elemental form, gas (7)
I believe the answer is:
ammonia
'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" .)