Ancient Ilium (4)
I believe the answer is:
troy
'ancient ilium' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for troy that I've seen before include "City of Paris" , "System of weights for measuring precious metals and gemstones" , "City besieged by the Greeks - system of weights for gems" , "Ancient city of the Iliad" , "Precious stone weight system" .)