Agent for change (8)
I believe the answer is:
catalyst
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">agent for change</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for catalyst that I've seen before include "Thing that causes a change" , "Chem. reaction enhancer" , "Something precipitating an event" , "Person or thing that precipitates change" , "Cause of change, itself unchanged" .)