Upper-class person of wisdom reveals custom (5)
I believe the answer is:
usage
'custom' is the definition.
(I know that custom can be written as usage)
'upper-class person of wisdom' is the wordplay.
'upper-class' becomes 'u' (similar in meaning).
'person of wisdom' becomes 'sage'.
'u'+'sage'='USAGE'
'reveals' acts as a link.
(Other definitions for usage that I've seen before include "Custom; practice" , "Usual practice" , "Interest of old" , "Customary practice" , "Employment - practice" .)