Possible Clues:

• Small songbird
• Small bird
• One flies to it via a small mountain
• Bird has to mount it
• Bird
• It takes a little bird to empty it, by the sound of it
• To empty it just a little 21 down
• Is a little bird going to empty it, by the sound of it? (6)
• A little bird tells me to empty it, by the sound of it
• A little bird tells me how to empty it, by the sound of it