Perfect place best one in posh area (6)
I believe the answer is:
utopia
'perfect place' is the definition.
(I've seen this before)
'best one in posh area' is the wordplay.
'best' becomes 'top' (top can mean the very best).
'one' becomes 'I' (Roman numeral).
'in' indicates putting letters inside.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'area' becomes 'a' (maths abbreviation).
'top'+'i'='topi'
'u'+'a'='ua'
'topi' inserted into 'ua' is 'UTOPIA'.
(Other definitions for utopia that I've seen before include "Place of perfect happiness" , "Sir Thomas Moore's perfect society" , "Impracticable state of perfection" , "Thomas More's perfect society" , "Imaginary place where everything is perfect" .)