Substance to curdle milk (6)
I believe the answer is:
rennet
'substance to curdle milk' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for rennet that I've seen before include "Substance used to curdle milk for cheese-making" , "cheesy setter?" , "Preparation for curdling milk" , "Causing vomiting" , "Cheese-making curdler" .)