City series in October Gazette backed (6)
I believe the answer is:
zagreb
'city' is the definition.
(largest city in Croatia)
'series in october gazette backed' is the wordplay.
'series' says the answer is hidden in the clue (I've seen 'series of' mean this (a series of letters from the clue)).
'backed' says the letters should be written backwards.
'bergaz' is hidden within 'in october gazette'.
'bergaz' backwards is 'ZAGREB'.
(Other definitions for zagreb that I've seen before include "Croatian city" , "Croatia's capital city" , "on River Sava" , "European capital" , "European city" .)