Upper-class fool in army group (4)
I believe the answer is:
unit
'army group' is the definition.
(thesaurus)
'upper-class fool' is the wordplay.
'upper-class' becomes 'u' ('u' can be similar in meaning to 'upper-class').
'fool' becomes 'nit' (I've seen this before).
'u'+'nit'='UNIT'
'in' is the link.
(Other definitions for unit that I've seen before include "Single complete domestic fixture" , "Quantity chosen as standard, or individual thing" , "Self-contained section" , "Single element; one" , "Individual thing as part of a whole" .)