One upper-class idiot (4)
I believe the answer is:
unit
'one' is the definition.
(synonyms)
'upper-class idiot' is the wordplay.
'upper-class' becomes 'u' (similar in meaning).
'idiot' becomes 'nit' (I've seen this before).
'u'+'nit'='UNIT'
(Other definitions for unit that I've seen before include "One part of a whole" , "detachment maybe" , "Module" , "Individual thing; one" , "Single item, maybe part of a set" .)