Posh idiot in military detachment (4)
I believe the answer is:
unit
'military detachment' is the definition.
(I've seen this in another clue)
'posh idiot' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'idiot' becomes 'nit' (nit can mean a stupid person).
'u'+'nit'='UNIT'
'in' acts as a link.
(Other definitions for unit that I've seen before include "Group that's part of a larger organisation" , "Detachment of soldiers" , "Standard for measuring" , "Item combining separate parts" , "One; military group" .)