African swaps volume for book about capital (7)

I believe the answer is:

nairobi

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'capital' is the definition.
(capital of Kenya)

I cannot understand how the rest of the clue works.

(Other definitions for nairobi that I've seen before include "African city" , "I, a robin (anag) - city" , "Kenyan city" , "City of Kenya" , "Foreign capital" .)

I've seen this clue in The Times.

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