Unbridled glee at unknown lament (5)
I believe the answer is:
elegy
'lament' is the definition.
(synonyms)
'unbridled glee at unknown' is the wordplay.
'unbridled' indicates an anagram (letters are unrestrained or unbridled).
'at' is a charade indicator (letters next to each other).
'unknown' becomes 'y' (algebra).
'glee' is an anagram of 'eleg'.
'eleg'+'y'='ELEGY'
(Other definitions for elegy that I've seen before include "Requiem" , "Sad words" , "Song of lament" , "Plaintive poem" , "Mournful song or poem" .)