Snake, when quiet (3)
I believe the answer is:
asp
'snake' is the definition.
(asp is a kind of snake)
'when quiet' is the wordplay.
'when' becomes 'as'.
'quiet' becomes 'p' (music symbol for 'piano').
'as'+'p'='ASP'
(Other definitions for asp that I've seen before include "creeper" , "Cleopatra-killing snake" , "This Egyptian killer" , "Snake, killed Cleopatra" , "One secreting poison" .)