Posh sun-hat adapted principally for an idyllic place (6)
I believe the answer is:
utopia
'an idyllic place' is the definition.
'posh sun-hat adapted principally' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'sun-hat' becomes 'topi' (type of Indian hat).
'principally' says to take the initial letters.
The first letter of 'adapted' is 'a'.
'u'+'topi'+'a'='UTOPIA'
'for' acts as a link.
(Other definitions for utopia that I've seen before include "Imaginary place where everything is perfect" , "Thomas More's perfect society" , "Thomas More's ideal state" , "More's imaginary state" , "Impracticable state of perfection" .)