Analyst's chemical (7)
I believe the answer is:
'analyst's chemical' is the definition.
(I've seen this in another clue)
This is the entire 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" .)