Active chemical substance (7)
I believe the answer is:
reagent
'active chemical substance' 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 "Chemical in reaction" , "Chemical substance used in tests" , "Substance with characteristic reactions, used as a chemical test" , "Chemically active substance" , "What causes change" .)