Socially acceptable kind of Green custom (5)
I believe the answer is:
usage
'custom' is the definition.
('usage' can be a synonym of 'custom')
'socially acceptable kind of green' is the wordplay.
'socially acceptable' becomes 'u'.
'kind of green' becomes 'sage'.
'u'+'sage'='USAGE'
(Other definitions for usage that I've seen before include "Interest of old" , "Customary practice" , "Terminology - custom" , "Convention" , "Way of treating" .)