Maybe Henry is Upper-class twit (4)
I believe the answer is:
unit
'maybe henry' is the definition.
(the Henry is a scientific unit)
'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'
'is' acts as a link.
(Other definitions for unit that I've seen before include "Single person - group of people" , "Body of soldiers" , "Group that's part of a larger organisation" , "Operational team" , "Whole - group" .)