Radioactive gas (5)
I believe the answer is:
radon
'radioactive gas' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for radon that I've seen before include "Element, symbol Rn" , "Inert gas that occurs naturally" , "A radioactive gas, symbol Rn" , "Natural gas considered a hazard to health" , "Gaseous element, Rn" .)