A modification accepted by both sides in Russian port (9)
I believe the answer is:
archangel
'russian port' is the definition.
(city and port in Archangelsk oblast, Russia)
'a modification accepted by both sides' is the wordplay.
'modification' becomes 'change' (synonyms).
'accepted by' means one lot of letters goes inside another (inserted letters are accepted).
'both sides' becomes 'RL' (abbreviations for right and left).
'change' put within 'rl' is 'rchangel'.
'a'+'rchangel'='ARCHANGEL'
'in' is the link.
(Other definitions for archangel that I've seen before include "Senior spirit; Russian port" , "Raphael, say" , "Among others, Michael" , "Port in northern Russia, or senior heavenly messenger" , "Gabriel, for example" .)