Dangerous noble gas (5)
I believe the answer is:
radon
'dangerous noble 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 "Rare noble gas" , "Radioactive gas, linked to granite" , "Inert gas - adorn (anag)" , "Noble gas, Rn" , "Natural gas considered a hazard to health" .)