Eg, argon (5,3)
I believe the answer is:
inert gas
'eg argon' is the definition.
(I know that Ar is a type of inert gas)
This is all the clue.
(Other definitions for inert gas that I've seen before include "provokes no reaction" , "He, amongst others" , "Eg, helium or argon" , "Such as neon or xenon" , "Eg, helium, xenon etc." .)