Custom socially acceptable to a wise man (5)
I believe the answer is:
usage
'custom' is the definition.
('usage' can be a synonym of 'custom')
'socially acceptable to a wise man' is the wordplay.
'socially acceptable' becomes 'u'.
'to a wise man' becomes 'sage' (sage is a kind of wise man).
'u'+'sage'='USAGE'
(Other definitions for usage that I've seen before include "Custom - treatment" , "Interest of old" , "Treatment; tradition" , "Customary way in which language is spoken or written" , "Acceptable language" .)