Watery part of milk (4)
I believe the answer is:
whey
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">watery part of milk</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for whey that I've seen before include "Watery content of milk" , "Part of 26 Across" , "Water separated from milk" , "Milk by-product" , "Residual liquid from clotted milk" .)