Watery liquid that separates from the curd when milk is clotted (4)
I believe the answer is:
whey
(Other definitions for whey that I've seen before include "Watery remains after the curds are separated in milk" , "Part of 26 Across" , "Watery content of milk" , "Watery constituent of milk" , "Part of Miss Muffet's diet" .)