Cheese-making by-product (4)
I believe the answer is:
whey
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">cheese-making by-product</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 whey that I've seen before include "Watery residue of milk" , "part of natural drink" , "Residue from junket" , "Watery part of milk after curd separation" , "Cheese-making by-product" .)