A numbered container? (3)

I believe the answer is:

box

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'container?' is the definition.
(I know that container can be written as box)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for box that I've seen before include "Spar; container" , "Tackle defender" , "Christmas present" , "Present container?" , "Engage in pugilism" .)

I've seen this clue in the Evening Standard.

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