Baking ingredient (5)
I believe the answer is:
yeast
'baking ingredient' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for yeast that I've seen before include "Brewing agent" , "Substance used in brewing and baking" , "Agent for making bread" , "that baker needs" , "Fungi used in bakery" .)