Customary practice of upper-class savant (5)
I believe the answer is:
usage
'customary practice' is the definition.
(I've seen this before)
'upper-class savant' is the wordplay.
'upper-class' becomes 'u' ('u' can be similar in meaning to 'upper-class').
'savant' becomes 'sage' (I've seen this before).
'u'+'sage'='USAGE'
'of' acts as a link.
(Other definitions for usage that I've seen before include "phraseology" , "Silly little thing" , "Interest of old" , "Habit" , "Usual practice" .)