Having an unreal lightness or delicateness (8)
I believe the answer is:
ethereal
'having an unreal lightness or delicateness' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ethereal that I've seen before include "'Extremely delicate, almost not of this world (8)'" , "Take ale there: it's light and delicate" , "Celestial; insubstantial" , "'Fairy-like, delicate (8)'" , "Fine" .)