Belgian city's vassal (5)
I believe the answer is:
liege
I believe this is a double definition.
'belgian city's' is the first definition.
(I've seen this before)
'vassal' is the second definition.
('liege' can be a synonym of 'vassal')
(Other definitions for liege that I've seen before include "City in eastern Belgium" , "Bound to render feudal service" , "Belgian city or feudal chief" , "Belgian city or ancient superior" , "European university" .)