Agent of change (8)
I believe the answer is:
catalyst
'agent of change' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for catalyst that I've seen before include "It initiates a chemical reaction without itself being affected" , "Something that aids change" , "Cause of change, itself unchanged" , "Chemical influencing change" , "Person or thing that precipitates change" .)