Upper-class twit constituent (4)
I believe the answer is:
unit
'constituent' is the definition.
(I've seen this before)
'upper-class twit' is the wordplay.
'upper-class' becomes 'u' (similar in meaning).
'twit' becomes 'nit' (both can mean a stupid person).
'u'+'nit'='UNIT'
(Other definitions for unit that I've seen before include "Element or part" , "detachment maybe" , "Single manufactured item; military grouping" , "Single item, maybe part of a set" , "- - trust" .)