Use most of milk supply for cereal (6)
I believe the answer is:
muesli
'cereal' is the definition.
(I've seen this before)
'use most of milk supply' is the wordplay.
'most of' means to remove the last letter.
'supply' is an anagram indicator (supply of letters to make).
'milk' with its final letter removed is 'mil'.
'use'+'mil'='usemil'
'usemil' anagrammed gives 'MUESLI'.
'for' acts as a link.
(Other definitions for muesli that I've seen before include "Breakfast food of cereal, fruits and nuts" , "Breakfast mixture of dry cereals and fruits" , "Breakfast mix of oats, nuts, fruit etc" , "Mixture of crushed grain, nuts etc" , "Breakfast cereal mix" .)