Bread leavener (5)
I believe the answer is:
yeast
'bread leavener' 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 "Fungus used to ferment sugar" , "Baker's ingredient" , "Baker's raising agent" , "Fungi used in bakery" , "Organism causing fermentation" .)