It initiates a chemical reaction without itself being affected (8)
I believe the answer is:
catalyst
'it initiates a chemical reaction without itself being affected' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for catalyst that I've seen before include "Something that aids change" , "Chem. reaction enhancer" , "It causes chemical change" , "Cause of change, itself unchanged" , "Substance improving a chemical reaction" .)