A case of share ownership? (9)

I believe the answer is:

portfolio

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'a case' is the definition.
(I know that portfolio is a type of case)

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

Can you help me to learn more?

(Other definitions for portfolio that I've seen before include "Large case" , "Cabinet responsibility" , "Set of investments held by one person" , "Collection of papers" , "artist's sample work" .)

I've seen this clue in The Irish Independent.

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