Relaxed state houses equally posh (6)
I believe the answer is:
casual
'relaxed' is the definition.
(I've seen this before)
'state houses equally posh' is the wordplay.
'state' becomes 'cal' (short for California).
'houses' is an insertion indicator.
'equally' becomes 'as' (synonyms).
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'as'+'u'='asu'
'cal' enclosing 'asu' is 'CASUAL'.
(Other definitions for casual that I've seen before include "Careless; unsystematic" , "Informal or blase" , "Relaxed; accidental" , "Of clothes, not formal" , "Irregular" .)