Catalyst (5)
'catalyst' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for agent that I've seen before include "who can act?" , "Instrument" , "Go-between" , "One who acts for another" , "One acting on behalf of another" .)
(Other definitions for reagent that I've seen before include "Chemical in reaction" , "Get near (anag) - substance used in chemistry" , "Chemical substance used in tests" , "will cause some reaction" , "Chemical with known properties" .)