Chemical substance is concerning spy (7)
I believe the answer is:
reagent
'chemical substance' is the definition.
(I know that reagent is a type of chemical substance)
'concerning spy' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'spy' becomes 'agent' (secret agent).
're'+'agent'='REAGENT'
'is' is the link.
(Other definitions for reagent that I've seen before include "What causes change" , "Chemical in reaction" , "Chemically-active substance" , "Chemical substance used in tests" , "Get near (anag) - substance used in chemistry" .)