Hazardous household gas (5)
I believe the answer is:
radon
'hazardous household 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 "Colourless gas, atomic number 86" , "hazardous substance" , "A radioactive gas, symbol Rn" , "Natural gas considered a hazard to health" , "Naturally occurring inert gas" .)