Substance used in chemical analysis (7)
I believe the answer is:
reagent
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">substance used in chemical analysis</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 reagent that I've seen before include "Substance with characteristic reactions, used as a chemical test" , "Chemically active substance" , "Get near (anag) - substance used in chemistry" , "What causes change" , "will cause some reaction" .)