Colourless gas with a chlorine-like odour, formed by an electric discharge in oxygen (5)
I believe the answer is:
ozone
(Other definitions for ozone that I've seen before include "Fresh, bracing sea air" , "Allotropic gas" , "lively atmosphere" , "Bracing air of old, now seen as potentially toxic" , "Kind of oxygen layer in atmosphere, has hole in it" .)