Subject: a Belgian city (5)
I believe the answer is:
liege
I believe this is a double definition.
'subject' is the first definition.
(thesaurus)
'a belgian city' is the second definition.
(I've seen this before)
(Other definitions for liege that I've seen before include "European province" , "Vassal, subject" , "World War one battle site" , "Belgian city or feudal superior" , "Belgian city - master - servant" .)