Like the noble gases (5)
I believe the answer is:
inert
'like the noble gases' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for inert that I've seen before include "Incapable of acting, like some gas" , "Passive, immobile" , "Unreacting" , "Unmoving" , "dead" .)