Snakes quietly tucking into animal (4)
I believe the answer is:
asps
'snakes' is the definition.
(asp is a kind of snake)
'quietly tucking into animal' is the wordplay.
'quietly' becomes 'p' (musical symbol).
'tucking into' indicates putting letters inside.
'animal' becomes 'ass' (term for a donkey).
'p' placed inside 'ass' is 'ASPS'.
(Other definitions for asps that I've seen before include "Deadly snakes" , "poisonous things" , "Cleopatra's snakes" , "Poisonous snakes" , "Poplars" .)