Setter's note rejected (6)
I believe the answer is:
rennet
'setter's' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'note rejected' is the wordplay.
'note' becomes 'tenner' (a tenner is a 10 pound note).
'rejected' is a reversal indicator.
'tenner' in reverse letter order is 'RENNET'.
Can you help me to learn more?
(Other definitions for rennet that I've seen before include "Preparation for curdling milk" , "Cheese-making curdler" , "It's used to curdle milk for cheese" , "Tenner (anag.); cheese ingredient" , "Dairy product" .)