A serpent when quiet (3)
I believe the answer is:
asp
'a serpent' is the definition.
(asp is a kind of serpent)
'when quiet' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'as'+'p'='ASP'
(Other definitions for asp that I've seen before include "This Egyptian killer" , "One secreting poison" , "Small poisonous snake" , "Kind of snake - what a sap" , "Cleopatra's supposed nemesis" .)