Substance used to curdle milk (6)
I believe the answer is:
rennet
'substance used to curdle milk' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rennet that I've seen before include "Milk curdling agent" , "Causing vomiting" , "substance used to make cheese" , "Preparation for curdling milk" , "Milk-curdling agent" .)