Spooner's kick up the backside for US drink (4,4)

I believe the answer is:

root beer

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'drink' is the definition.
(root beer is a kind of drink)

I don't understand the rest of the clue.

(Other definitions for root beer that I've seen before include "Mostly American drink" , "Drink (mostly in US)" , "Effervescent drink" , "Fizzy drink (mainly US)" , "American soft drink" .)

I've seen this clue in The Guardian.

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