Like neon (5)
I believe the answer is:
inert
'like neon' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for inert that I've seen before include "Unable to move or resist motion" , "Devoid of motion or change" , "Sluggish - unresponsive" , "Lifeless; apathetic" , "Not chemically reactive" .)