Unreactive form of nitre (5)
I believe the answer is:
inert
'unreactive' is the definition.
(similar in meaning)
'form of nitre' is the wordplay.
'form of' is an anagram indicator (another form of the same letters).
'nitre' with letters rearranged gives 'INERT'.
(Other definitions for inert that I've seen before include "lazy" , "Unreacting" , "dead" , "Sluggish, inactive" , "Lifeless; apathetic" .)