Leaven (5)
I believe the answer is:
yeast
'leaven' is the definition.
(I know that yeast is a type of leaven)
This is the entire clue.
(Other definitions for yeast that I've seen before include "Androgynous" , "Baker's ingredient" , "Leavening agent in bread-making" , "Dough raising agent" , "Rising agent" .)