Sullen, introducing new poem of lament (6)
I believe the answer is:
monody
'poem of lament' is the definition.
'monody' can be an answer for 'lament' (I've seen this before). I am not sure about the 'poem of' bit.
'sullen introducing new' is the wordplay.
'sullen' becomes 'moody' ('moody' can be a synonym of 'sullen').
'introducing' means one lot of letters goes inside another (some letters introduced or inserted into others).
'new' becomes 'n'.
'moody' going around 'n' is 'MONODY'.
Can you help me to learn more?
(Other definitions for monody that I've seen before include "Plainchant, perhaps" , "musical style" , "elegy" , "Song that's mournful" , "kind of poem" .)