It goes to one's head (3)

I believe the answer is:

cap

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'head' is the definition.
(both can mean the top of something)

I don't understand the rest of the clue.

(Other definitions for cap that I've seen before include "Top; hat" , "Flat brimless hat" , "Seal off oil-well" , "Soft headgear" , "International sportsman" .)

I've seen this clue in the Evening Standard.

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