A snake appears so quietly (3)
I believe the answer is:
asp
'a snake appears' is the definition.
(I've seen this before)
'so quietly' is the wordplay.
'so' becomes 'as' (eg 'not as tall as me' means 'not so tall as me').
'quietly' becomes 'p' (p for piano and quiet).
'as'+'p'='ASP'
(Other definitions for asp that I've seen before include "something at the zoo" , "Cleopatra's poisoner" , "Cold-blooded killer" , "Predatory fish" , "Small poisonous snake" .)