Odorless gas (5)
I believe the answer is:
radon
'odorless 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 "Radioactive gas, linked to granite" , "Inert gas - adorn (anag)" , "hazardous substance" , "Noble gas, Rn" , "Naturally occurring inert gas" .)