Surprisingly nitre is unreactive (5)
I believe the answer is:
inert
'unreactive' is the definition.
('inert' can be similar in meaning to 'unreactive')
'surprisingly nitre' is the wordplay.
'surprisingly' indicates an anagram.
'nitre' anagrammed gives 'INERT'.
'is' is the link.
(Other definitions for inert that I've seen before include "Dormant, lifeless" , "Chemically inactive" , "Passive, immobile" , "Sluggish, inactive" , "Not chemically active" .)