As quiet as a snake (3)
I believe the answer is:
asp
'a snake' is the definition.
(asp is a kind of snake)
'as quiet' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'as'+'p'='ASP'
'as' acts as a link.
(Other definitions for asp that I've seen before include "Cleopatra's poisoner" , "Snake of the Nile" , "something at the zoo" , "Kind of snake - what a sap" , "One secreting poison" .)