Advanced piece of music, possibly burdensome (4)
I believe the answer is:
onus
'advanced piece' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'of music possibly burdensome' is the wordplay.
I cannot quite see how this works, but
'of' could be 'o' and 'o' is present in the answer.
The remaining letters 'nus' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for onus that I've seen before include "'Responsibility, task or burden (4)'" , "'Burden, of proof (4)'" , "Encumbrance" , "albatross?" , "Responsibilty" .)