Substance aiding a reaction (8)
I believe the answer is:
catalyst
'substance aiding a reaction' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for catalyst that I've seen before include "Person or thing that precipitates change" , "Something precipitating an event" , "Chem. reaction enhancer" , "Chemical influencing change" , "It initiates a chemical reaction without itself being affected" .)