A container to open and shut? (4)

I believe the answer is:

case

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'a container' is the definition.
(I know that container can be written as case)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for case that I've seen before include "Police investigation" , "Overnight bag" , "Reconnoitre" , "Keep careful watch on" , "See 12" .)

I've seen this clue in the Evening Standard.

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