Piece of luggage, one for Sherlock? (4)

I believe the answer is:

case

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'piece of luggage' is the definition.
'case' can be an answer for 'luggage' (I've seen this in other clues). I am not sure about the 'piece of' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for case that I've seen before include "proceedings" , "Check out" , "Instance - lawsuit" , "Carefully inspect < suit" , "Patient - luggage" .)

I've seen this clue in The Guardian.

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