Curdled milk, note, returned (6)
I believe the answer is:
rennet
'curdled milk' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'note returned' is the wordplay.
'note' becomes 'tenner' (a ten pound sterling bank note).
'returned' says the letters should be written backwards.
'tenner' backwards is 'RENNET'.
Can you help me to learn more?
(Other definitions for rennet that I've seen before include "Substance that curdles milk" , "Cheesemakers use this" , "Cheese curdling agent" , "what you need to make cheese" , "Cheese-maker's substance" .)