Shape US defence is in? (8)

I believe the answer is:

pentagon

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'shape' is the definition.
(pentagon is a kind of shape)

I cannot understand how the remainder of the clue works.

(Other definitions for pentagon that I've seen before include "US building of high security" , "US military leaders" , "Five-sided polygon" , "government HQ abroad" , "Figure - US military HQ" .)

I've seen this clue in The Sun.

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