But it doesn't stretch to music (6,4)
I believe the answer is:
rubber band
'music' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for rubber band that I've seen before include "Elasticated loop" , "Stretchy fastener" , "use it to hold things together" , "Elastic item" , "Some bridge players" .)