Head of Allies taps Reichstag declaration (3)

I believe the answer is:

ach

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'reichstag declaration' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'head of allies taps' is the wordplay.
'head of' suggests taking the first letters.
'taps' becomes 'ch' (cold and hot taps).
The first letter of 'allies' is 'a'.
'a'+'ch'='ACH'

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I've seen this clue in the Sydney Morning Herald.

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