Agent for change (8)
I believe the answer is:
catalyst
'agent for change' is the definition.
(I've seen this before)
This is all the clue.
(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" .)