Declared method used would produce a watery liquid (4)
I believe the answer is:
whey
'a watery liquid' is the definition.
(I've seen this before)
'declared method' is the wordplay.
'declared' indicates a 'sounds like' (homophone) clue.
'method' becomes 'way' (way of doing something).
'way' sounds like 'WHEY'.
'used would produce' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for whey that I've seen before include "Watery liquid left when milk forms curds" , "Watery stuff from clotted milk" , "Cheese-making by-product" , "part of natural drink" , "Milk by-product" .)