Brown takes in top class quietly, the snake (6)
I believe the answer is:
taipan
'the snake' is the definition.
(taipan is a kind of snake)
'brown takes in top class quietly' is the wordplay.
'brown' becomes 'tan' (tan is a kind of brown).
'takes in' means one lot of letters goes inside another.
'top class' becomes 'ai' (I've seen this in other clues).
'quietly' becomes 'p' (p for piano and quiet).
'ai'+'p'='aip'
'tan' going around 'aip' is 'TAIPAN'.
(Other definitions for taipan that I've seen before include "Large brown, highly-venomous snake" , "Large Australian snake" .)