Crossword Genius

How formula for poison is written in capital city (6)

Ross

I believe the answer is:

athens

'capital city' is the definition.
(capital of Greece)

I don't understand how the rest of the clue works.

(Other definitions for athens that I've seen before include "Hasten (anag) -- capital city" , "Hasten to Greek city" , "city in Greece" , "City in which 13 Across founded school" , "EU capital" .)

I've seen this clue in The Guardian.
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