Dangerous household gas (5)
I believe the answer is:
radon
'dangerous household gas' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for radon that I've seen before include "Gaseous element, Rn" , "Rare noble gas" , "Colourless gas, atomic number 86" , "Naturally occurring inert gas" , "Element, symbol Rn" .)