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