Upset nuts pack in a city (7)

I believe the answer is:

potsdam

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'city' is the definition.
(I have seen 'German city' mean 'potsdam' so perhaps 'city' could also mean 'potsdam')

'upset nuts pack' is the wordplay.
'upset' shows that the letters should be reversed in order.
'nuts' becomes 'mad' (informal term).
'pack' becomes 'stop' (I can't explain this - if you can you should give a lot more credence to this answer).
'mad'+'stop'='madstop'
'madstop' written backwards gives 'POTSDAM'.

'in a' is the link.

Can you help me to learn more?

(Other definitions for potsdam that I've seen before include "WW two 1945 conference; stop mad (anag.)" , "where conference was held" , "City near Berlin, site of Sanssouci palace" , "Site of nineteen forty fve conference" , "conference city" .)

I've seen this clue in The Daily Express.

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