Of such music, a piece is of questionable length (6)
I believe the answer is:
string
'of such music' is the definition.
The definition suggests a singular noun which matches the answer.
'a piece is of questionable length' is the wordplay.
I cannot really see how this works, but
'piece' could be 'ring' (I have seen 'Piece of jewellery ' mean 'ring' so perhaps 'piece' could also mean 'ring') and 'ring' is present in the answer.
The remaining letters 'st' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for string that I've seen before include "Number of racehorses" , "train" , "Violin wire" , "Thread; sequence" , "Piece of catgut stretched on violin" .)