Posh and wise management (5)
I believe the answer is:
usage
'management' is the definition.
(I've seen this before)
'posh and wise' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'and' says to put letters next to each other.
'wise' becomes 'sage' ('sage' can be similar in meaning to 'wise').
'u'+'sage'='USAGE'
(Other definitions for usage that I've seen before include "Customary practice" , "Custom - treatment" , "Habitual custom" , "Treatment, practice" , "etiquette" .)