Old city's endless conflict in past (6)
I believe the answer is:
bombay
'old city's' is the definition.
(former name of Mumbai)
'endless conflict in past' is the wordplay.
'endless' indicates the central letters.
'conflict' becomes 'combat' (combat is a kind of conflict).
'in' is an insertion indicator.
'past' becomes 'by' (as in years gone by).
The middle of 'combat' is 'omba'.
'omba' placed into 'by' is 'BOMBAY'.
(Other definitions for bombay that I've seen before include "Former name of Indian city - kind of duck?" , "Indian city sounds like deadly part of aircraft" , "old city" , "Indian port" , "Former name of a city and port in the west of India" .)