Northern city half filled with superior old people (6)
I believe the answer is:
manchu
'old people' is the definition.
(Chinese ethnic group)
'northern city half filled with superior' is the wordplay.
'northern city' becomes 'Manchester' (city in the northwest of England).
'half filled' indicates taking half.
'with' says to put letters next to each other.
'superior' becomes 'U' (U can mean upper-class, as in U and non-U speech).
'manchester' with half the letters taken is 'manch'.
'manch'+'u'='MANCHU'
(Other definitions for manchu that I've seen before include "Chinese dynasty overthrown in 1911" , "Last imperial dynasty of China" , "Last Chinese imperial dynasty" , "old language" , "Seventeenth century conquerors of China" .)