As spelt out in foreign capital (6)
I believe the answer is:
athens
'foreign capital' is the definition.
'Athens' can be an answer for 'capital' (capital of Greece). I am not certain of the 'foreign' bit.
'as spelt out in' is the wordplay.
'spelt out' becomes 'then' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'as' enclosing 'then' is 'ATHENS'.
Can you help me to learn more?
(Other definitions for athens that I've seen before include "Greek capital" , "Hasten to European city" , "city in Greece" , "Site of the Parthenon" , "foreign city" .)