Note the appearance of upper-class people (6)
I believe the answer is:
gentry
'people' is the definition.
(upper-class people)
'note the appearance of upper-class' is the wordplay.
'note' becomes 'g' (musical note).
'the appearance of upper class' becomes 'entry' (I can't justify this - if you can you should believe this answer much more).
'g'+'entry'='GENTRY'
Can you help me to learn more?
(Other definitions for gentry that I've seen before include "Minor aristocracy" , "People of a social position just below the nobility" , "Upper classes" , "People of high birth" , "Class next below nobility" .)