Ancient besieged city (4)
I believe the answer is:
troy
'ancient besieged city' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for troy that I've seen before include "System of weights introduced by French" , "Wooden horse siege city" , "See 7" , "City impacted by long-term investment" , "part of Asia Minor" .)