Its contents go to one's head (6)

I believe the answer is:

hatbox

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'head' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps you can see a link between them that I don't see?

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

Can you help me to learn more?

(Other definitions for hatbox that I've seen before include "Case for toppers" , "Case for storing headgear" , "Container for headwear" , "Milliner's container" , "Ten-gallon container maybe" .)

I've seen this clue in the Evening Standard.

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