How the US voted encompasses Donald's number one taboo (9)

I believe the answer is:

forbidden

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'one taboo' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for forbidden that I've seen before include "Prohibited, taboo" , "Illicit pleasure" , "Disallowed, banned" , "Verboten, prohibited" , "Not allowed, banned" .)

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