Have or haven't got years to make leaven (5)
I believe the answer is:
yeast
'leaven' is the definition.
(I know that yeast is a type of leaven)
'have or haven't got years' is the wordplay.
I cannot really understand how this works, but
'years' could be 'y' (abbreviation) and 'y' is found in the answer.
The remaining letters 'east' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'to make' is the link.
Can you help me to learn more?
(Other definitions for yeast that I've seen before include "Bread-making component" , "Rising agent" , "lifting agent" , "Brewing agent" , "A raising agent in baking" .)