Design at each posh castle (7)
I believe the answer is:
chateau
'castle' is the definition.
(thesaurus)
'design at each posh' is the wordplay.
'design' indicates anagramming the letters.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'at'+'each'='ateach'
'ateach' with letters rearranged gives 'chatea'.
'chatea'+'u'='CHATEAU'
(Other definitions for chateau that I've seen before include "French manor house" , "eg Margaux" , "French county house" , "Large country house in France" , "Impressive dwelling in France" .)