Ideal society finest one featured in posh article (6)
I believe the answer is:
utopia
'ideal society' is the definition.
'finest one featured in posh article' is the wordplay.
'finest' becomes 'top' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'featured in' is an insertion indicator.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'article' becomes 'a' (indefinite article).
'top'+'i'='topi'
'u'+'a'='ua'
'topi' put within 'ua' is 'UTOPIA'.
(Other definitions for utopia that I've seen before include "Eden" , "More's ideal world" , "Imagined state of perfection" , "Ideal but unattainable state" , "Impracticable state of perfection" .)