Makes fun of heaven in unreasonable delight (5,8)
I believe the answer is:
fools paradise
'unreasonable delight' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'makes fun of heaven' is the wordplay.
'makes fun of' becomes 'fools' (I can't justify this - if you can you should give a lot more credence to this answer).
'heaven' becomes 'paradise' (synonyms).
'fools'+'paradise'='FOOLS PARADISE'
'in' is the link.
Can you help me to learn more?
(Other definitions for fools paradise that I've seen before include "Happy state while ignorant of potential trouble" , "Blissfully ignorant state" , "Place of happy denial" , "Deceptively happy state" , "Illusory happiness" .)