Byron's end portrayed in melancholy lament (6)
I believe the answer is:
monody
'lament' is the definition.
(mournful song in ancient Greek drama)
'byron's end portrayed in melancholy' is the wordplay.
'end' says to take the final letters.
'portrayed in' is an insertion indicator.
'melancholy' becomes 'moody'.
The final letter of 'byron' is 'n'.
'n' placed inside 'moody' is 'MONODY'.
(Other definitions for monody that I've seen before include "elegy" , "Song that's mournful" , "Plainchant, perhaps" , "musical style" , "lament" .)