Harmful gas
I believe the answer is:
radon
'harmful gas' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for radon that I've seen before include "Gaseous element, Rn" , "hazardous substance" , "gets no reaction" , "Natural gas considered a hazard to health" , "Colourless gas, atomic number 86" .)