Leavening agent (5)
I believe the answer is:
yeast
'leavening agent' 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 "that baker needs" , "fermenter" , "A raising agent in baking" , "Fungus used to ferment sugar" , "Baker's ingredient" .)