Capital letter from Argos followed by one from Sparta (6)
I believe the answer is:
athens
'capital' is the definition.
(capital of Greece)
'letter from argos followed by one from sparta' is the wordplay.
'letter from argos' becomes 'a' (I can't explain this - if you can you should believe this answer much more).
'followed by' becomes 'then' (I've seen this in another clue).
'one from' indicates taking the first letters.
The first letter of 'sparta' is 's'.
'a'+'then'+'s'='ATHENS'
Can you help me to learn more?
(Other definitions for athens that I've seen before include "Hasten to Greece" , "Plato's birthplace" , "Hasten (anag) -- capital city" , "Site of the Parthenon" , "Hen sat down in the city" .)