Crossword Genius

Dairy cow jumper (8)

Ross

I believe the answer is:

guernsey

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

This clue could be a double definition.

'dairy cow' is the first definition.
(Guernsey is an example)

'jumper' is the second definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

Can you help me to learn more?

(Other definitions for guernsey that I've seen before include "Source of milk" , "Lyric poem" , "Second-largest Channel Island" , "Second largest Channel Island" , "Lower" .)

I've seen this clue in the Sydney Morning Herald.
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