Fast food for German citizens (10)

I believe the answer is:

hamburgers

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'german citizens' is the definition.
Although both the answer and definition are plural nouns, I cannot understand how one could define the other.

'fast food for' is the wordplay.
'fast food' becomes 'hamburger' (I've seen this before).
'for' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'hamburger'+'s'='HAMBURGERS'

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