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 located in the answer.
The remaining letters 'st' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
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" .)