Producer of milk with thick top (6)

I believe the answer is:

jersey

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'top' is the definition.
(item of clothing)

I can't explain the remainder of the clue.

(Other definitions for jersey that I've seen before include "Largest of the Channel Islands" , "farm animal" , "whose yellow variety represents the cream" , "Supplier of dairy produce" , "One of the Channel Islands" .)

I've seen this clue in The Guardian.

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