Eternal fool has unbridled glee (7)
I believe the answer is:
ageless
'eternal' is the definition.
(I know that eternal can be written as ageless)
'fool has unbridled glee' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'has' is an insertion indicator.
'unbridled' indicates an anagram (letters are unrestrained or unbridled).
'glee' with letters rearranged gives 'gele'.
'ass' placed around 'gele' is 'AGELESS'.
(Other definitions for ageless that I've seen before include "Eternally young" , "Eternal - like Peter Pan" , "Ever young" , "Enduring" , "unaffected by time" .)