Silly upper-class name? Ernie, perhaps (6)
I believe the answer is:
unwise
'silly' is the definition.
(thesaurus)
'upper-class name? ernie perhaps' is the wordplay.
'upper-class' becomes 'u' (similar in meaning).
'name?' becomes 'n' (common abbreviation e.g. on forms).
'ernie perhaps' becomes 'Wise' (comedian Ernie Wise).
'u'+'n'+'wise'='UNWISE'
(Other definitions for unwise that I've seen before include "Ill-judged" , "isn't sensible" , "Not sensible, foolish" , "Rash" , "Not sensible or prudent" .)