Illusory happiness (5,8)
I believe the answer is:
fools paradise
'illusory happiness' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for fools paradise that I've seen before include "Blissfully ignorant state" , "Illusory happiness" , "Ill-founded state of happiness" , "State of ignorant happiness" , "Place of happy denial" .)