Container capable of expansion (5)

I believe the answer is:

chest

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'container' is the definition.
(I know that chest is a type of box)

I can't explain the remainder of the clue.

(Other definitions for chest that I've seen before include "Large, sturdy box with a lid" , "Treasure (or heart) container" , "Box - breast" , "Large box; thorax" , "Upper trunk" .)

I've seen this clue in the Evening Standard.

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