What sheep might have sheared off top (4)

I believe the answer is:

horn

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'top' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe you can see a link between them that I don't see?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for horn that I've seen before include "Head growth; cornucopia" , "Cusp" , "Musical instrument; Chile Cape" , "Hollow outgrowth and musical instrument" , "Bony outgrowth on an animal" .)

I've seen this clue in The Guardian.

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