Milk serum (4)
I believe the answer is:
whey
'milk serum' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for whey that I've seen before include "Residue from junket" , "Watery dairy product" , "Milk by-product" , "Watery stuff from clotted milk" , "Watery liquid left when milk forms curds" .)