City circles infiltrated by the Left (6)
I believe the answer is:
oporto
'city' is the definition.
(city in Portugal)
'circles infiltrated by the left' is the wordplay.
'circles' becomes 'oo' (two O's - O resembles a circle).
'infiltrated by' indicates putting letters inside.
'the left' becomes 'port' (left side of a ship - opposite of starboard).
'oo' going around 'port' is 'OPORTO'.
(Other definitions for oporto that I've seen before include "Portuguese wine trade centre" , "Wine city in Portugal" , "Second largest city in Portugal" , "European city" , "City near the mouth of the River Douro" .)