Unbridled glee at unknown lament (5)
I believe the answer is:
elegy
'lament' is the definition.
(synonyms)
'unbridled glee at unknown' is the wordplay.
I cannot quite see how this works, but
an anagram of 'glee' is 'eleg' which is present in the answer.
'unknown' could be 'y' (algebra) and 'y' is found in the answer.
No letters remain.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for elegy that I've seen before include "Requiem" , "Sad words" , "Song of lament" , "Plaintive poem" , "Mournful song or poem" .)