Poet turning tail, cause of ferment? (5)
I believe the answer is:
yeast
'ferment?' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for yeast that I've seen before include "Fungi used in bakery" , "that baker needs" , "lifting agent" , "Dough-raising agent" , "It makes dough rise" .)