Cast ultimately drowned out by quiet beat (6)
I believe the answer is:
thresh
'beat' is the definition.
(I know that thresh is a kind of beat)
'cast ultimately drowned out by quiet' is the wordplay.
I cannot quite see how this works, but
'quiet' could be 'sh' (both can be said to tell someone to be quiet) and 'sh' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for thresh that I've seen before include "Extract grain (with flail)" , "Beat grain or make violent movements" , "Separate corn from husks by beating" , "Beat (stalks to get grain)" , "Separate grain from (corn); move violently" .)