Foreign city left with zero protection either side (6)
I believe the answer is:
oporto
'foreign city' is the definition.
(city in Portugal)
'left with zero protection either side' is the wordplay.
I cannot quite understand how this works, but
'left' could be 'port' (left side of a ship - opposite of starboard) and 'port' is located in the answer.
'zero' could be 'o' (O resembles 0) and 'o' is present in the answer.
A single letter 'o' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for oporto that I've seen before include "Iberian location" , "Wine city in Portugal" , "Portugal's second city" , "Portuguese wine trade centre" , "City near the mouth of the River Douro" .)