Analyst's chemical (7)
I believe the answer is:
reagent
'analyst's chemical' 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" , "What causes change" , "Active chemical" , "Chemically active substance" , "Chemically-active substance" .)