About 1% of the atmosphere (5)
I believe the answer is:
argon
'about 1% of the atmosphere' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for argon that I've seen before include "Inert gas, symbol Ar" , "17 [GAS]" , "Gas used in electric lights" , "noble element" , "Inert gas in atmosphere" .)