Substance used in chemical analysis (7)
I believe the answer is:
reagent
'substance used in chemical analysis' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for reagent that I've seen before include "Substance with characteristic reactions, used as a chemical test" , "Chemically active substance" , "Get near (anag) - substance used in chemistry" , "What causes change" , "will cause some reaction" .)