Capital in those days invested in arsenic (6)
I believe the answer is:
athens
'capital' is the definition.
(capital of Greece)
'in those days invested in arsenic' is the wordplay.
'in those days' becomes 'then' (I've seen this in another clue).
'invested in' means one lot of letters goes inside another.
'arsenic' becomes 'As' (As is the chemical symbol for arsenic).
'then' placed into 'as' is 'ATHENS'.
(Other definitions for athens that I've seen before include "foreign city" , "Hen sat down in the city" , "city in Greece" , "Two thousand and four Olympics city" , "Hasten to Greek city" .)