Possibly Irving’s capital? (6)

I believe the answer is:

berlin

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'capital?' is the definition.
(capital of Germany)

I don't understand the remainder of the clue.

(Other definitions for berlin that I've seen before include "German capital" , "Isaiah, say?" , "City once divided by wall" , "Composer of White Christmas" , "German city" .)

I've seen this clue in The Times.

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