Unpleasant-smelling colourless gas (7)

I believe the answer is:

ammonia

(Other definitions for ammonia that I've seen before include "Pungent gas formed of nitrogen and hydrogen" , "Gas compounded of nitrogen and hydrogen, NH3" , "Colourless gas with strong smell" , "Powerful cleaner" , "Pungent cleaning fluid" .)

I've seen this clue in The Times.

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