Substance to curdle milk (6)
I believe the answer is:
rennet
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">substance to curdle milk</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)