Ancient city; system of weights (4)
I believe the answer is:
troy
This clue is a double definition.
'ancient city' is the first definition.
(Troy was an ancient city)
'system of weights' is the second definition.
(troy is a kind of system of weights)
(Other definitions for troy that I've seen before include "Ancient city; type of weight" , "Wooden horse siege city" , "Ancient city of the Iliad" , "System of weights for precious stones and metals" , "Iliad siege city" .)