Fermentation agent
I believe the answer is:
yeast
'fermentation agent' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for yeast that I've seen before include "What makes bread rise" , "Substance used to raise dough etc" , "Bread-making component" , "As yet (anag)" , "Dough raising agent" .)