Knowledge test for the upper classes (6)
I believe the answer is:
gentry
'the upper classes' is the definition.
(I've seen this before)
'knowledge test' is the wordplay.
'knowledge' becomes 'gen' (I've seen this before).
'test' becomes 'try' (synonyms).
'gen'+'try'='GENTRY'
'for' is the link.
(Other definitions for gentry that I've seen before include "People of high birth" , "People of good standing" , "People of a social position just below the nobility" , "people of breeding show" , "Nobs" .)