It’s round and usually compact (4)

I believe the answer is:

disc

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'it's round' is the definition.
(disc is a kind of round)

I don't understand how the rest of the clue works.

(Other definitions for disc that I've seen before include "Protein-constituent acid" , "put on by jockey?" , "Thin circular plate, as in CD" , "something moving in the spine" , "Capital of India" .)

I've seen this clue in the Evening Standard.

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