Noble gas, Rn (5)
I believe the answer is:
radon
This clue is a double definition.
'noble gas' is the first definition.
(I've seen this before)
'rn' is the second definition.
(I know that rn can be written as radon)
(Other definitions for radon that I've seen before include "hazardous substance" , "Colourless gas, atomic number 86" , "Gaseous element, Rn" , "Radioactive gas, linked to granite" , "Inert gas that occurs naturally" .)