An inert gas (5)
I believe the answer is:
argon
'an inert gas' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for argon that I've seen before include "Inert gas, symbol Ar" , "Inert eighteen ac gas" , "rare gas" , "Noble gas used in light bulbs" , "Noble gas, atomic number 18" .)