Thank you, kind sir.

Event for quiet, grown-up chick - sign on (10)

[I usually start with the direct when making a cryptic. ]

Phenomenon

Event = direct ; quiet = p; grown up chick = hen; sign = omen, + on

Awesome clue, Pandora. I wrestled with it because once I figured the first letter had to be that P, I still couldn't see how.
As a piano student, I should have known.