Nonreactive, like neon and argon (5)
I believe the answer is:
inert
I believe this is a double definition.
'nonreactive' is the first definition.
(I've seen this in another clue)
'like neon and argon' is the second definition.
(I've seen this in another clue)
(Other definitions for inert that I've seen before include "Chemically unreactive" , "Sluggish, still" , "Not chemically active" , "Sluggish, inactive" , "Inter, being unable to move" .)