Nitre treated like nitrogen (5)
I believe the answer is:
inert
'like nitrogen' is the definition.
(I've seen this before)
'nitre treated' is the wordplay.
'treated' is an anagram indicator.
'nitre' with letters rearranged gives 'INERT'.
(Other definitions for inert that I've seen before include "Like e.g. xenon" , "(Chem.) not reactive" , "Incapable of acting, like some gas" , "Not chemically reactive" , "Motionless, still" .)