Organic catalyst (6)
I believe the answer is:
enzyme
'organic catalyst' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for enzyme that I've seen before include "Protein produced by living cells" , "Protein product of living cells" , "It acts as a catalyst to bring about biochemical reaction" , "Protein substance acting as a catalyst" , "Substance created in living cells acting as a catalyst" .)