Supplier of milk reduced drink (3)
I believe the answer is:
tea
'drink' is the definition.
(tea is a kind of drink)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for tea that I've seen before include "(Infused) drink; meal" , "Refreshment" , "Dried leaf infusion" , "Oolong, for example" , "Elevenses" .)