Ambushes set up next to a city long ago (6)
I believe the answer is:
sparta
'city long ago' is the definition.
(ancient city)
'ambushes set up next to a' is the wordplay.
'ambushes' becomes 'traps' ('trap' can be a synonym of 'ambush').
'set up' shows that the letters should be reversed in order.
'next to' is a charade indicator (letters next to each other).
'traps' back-to-front is 'spart'.
'spart'+'a'='SPARTA'
(Other definitions for sparta that I've seen before include "Ancient kingdom renowned for asceticism" , "austere place" , "found in Ancient Greek location" , "Old Greek city state, famous for its citizens' austere lifestyle" , "Ancient Greek city noted for tough citizens" .)