Ideal but unattainable state (6)
I believe the answer is:
utopia
'ideal but unattainable state' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for utopia that I've seen before include "Imaginary ideal place" , "Idealised place" , "A perfect place according to Thomas More" , "Sir Thomas More's perfect place" , "Sir Thomas More's perfect country" .)