Russian port's leading backer (9)
I believe the answer is:
archangel
'russian port's' is the definition.
(city and port in Archangelsk oblast, Russia)
'leading backer' is the wordplay.
'leading' becomes 'arch' (e.g. in arch-enemy).
'backer' becomes 'angel' ('angel' can be a synonym of 'backer').
'arch'+'angel'='ARCHANGEL'
(Other definitions for archangel that I've seen before include "Russian city" , "Highest ranking member of the celestial hierarchy" , "Among others, Michael" , "Nag Rachel (anag.)" , "Senior heavenly messenger or Russian port" .)