Capital at that time invested in a Society (6)
I believe the answer is:
athens
'capital' is the definition.
(Athens is an example)
'that time invested in a society' is the wordplay.
'that time' becomes 'then'.
'invested in' is an insertion indicator.
'society' becomes 's' (common abbreviation in names of organisations).
'a'+'s'='as'
'then' inserted within 'as' is 'ATHENS'.
'at' acts as a link.
(Other definitions for athens that I've seen before include "Plato's birthplace" , "Hasten to European city" , "Hasten to Greek city" , "European capital city" , "Capital of Greece" .)