Sir, in French or Belgian city; that is an ancient city (8)
I believe the answer is:
monsieur
'sir in french' is the definition.
The definition and answer can be both people as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'belgian city that is an ancient city' is the wordplay.
'belgian city' becomes 'mons' (I've seen this in another clue).
'that is' becomes 'i.e.' (short for Latin 'id est', 'that is').
'an ancient city' becomes 'ur'.
'mons'+'ie'+'ur'='MONSIEUR'
'or' is the link.
Can you help me to learn more?
(Other definitions for monsieur that I've seen before include "French gentleman" , "French man" , "Frenchman" , "French title of respect" , "Frenchman's title" .)