Old city in Belgium with gold agency (6)
I believe the answer is:
bureau
'agency' is the definition.
('bureau' can be a synonym of 'agency')
'old city in belgium with gold' is the wordplay.
'old city' becomes 'ur' (ancient city state).
'in' indicates putting letters inside.
'belgium' becomes 'b' (international vehicle registration code).
'with gold' becomes 'eau' (I can't justify this - if you can you should believe this answer much more).
'b'+'eau'='beau'
'ur' put within 'beau' is 'BUREAU'.
Can you help me to learn more?
(Other definitions for bureau that I've seen before include "Piece of furniture with drawers" , "Writing desk with drawers" , "Wtiting desk" , "Item of furniture" , "Room" .)