This one is pretty simple, All you must do to recieve the clarinet is complete level 5. After defeating the boss at the end the chest will appear with the clarinet inside.
Quick tip for the boss: just run back and fourth infront of him to make him slam his hands down. After he misses you twice he will throw his hands in the air and a target will appear in his mouth just spin at it and hit it. do this 5-6 times and he will go down
thanks and good luck,
artyman60