Capital at that time invested in cash essentially (6)
I believe the answer is:
athens
'capital' is the definition.
(capital of Greece)
'that time invested in cash essentially' is the wordplay.
'that time' becomes 'then' (at some former time).
'invested in' indicates putting letters inside.
'essentially' means to look at the middle letters.
The middle letters of 'cash' are 'as'.
'then' put inside 'as' is 'ATHENS'.
'at' is the link.
(Other definitions for athens that I've seen before include "6 capital" , "city in Greece" , "Tan she got in Greece" , "Greek capital" , "Hen sat down in the city" .)