Colourless pungent gas (7)
I believe the answer is:
ammonia
'colourless pungent gas' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ammonia that I've seen before include "Smelling salts (NH3)" , "Gas; cleaning fluid" , "Gas released by smelling salts" , "Gas compounded of nitrogen and hydrogen, NH3" , "pungent chemical" .)