Dairy product possibly by the cup (6)

I believe the answer is:

butter

I'm a little stuck... Click here to teach me more about this clue!

'dairy product' is the definition.
(butter is a kind of dairy product)

I don't understand the remainder of the clue.

(Other definitions for butter that I've seen before include "Dairy product" , "Bread-spread made from milk" , "& 22 Mincepie accompaniment" , "Spread derived from cream" , "Milk product" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app