Watery part of milk (4)
I believe the answer is:
whey
'watery part of milk' is the definition.
(I've seen this before)
This is all the clue.
(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" .)