Catalyst (5)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">catalyst</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)