Beat trio, almost quiet (6)
I believe the answer is:
thresh
'beat' is the definition.
(threshing is a kind of beating)
'trio almost quiet' is the wordplay.
'trio' becomes 'three' ('three' can be a synonym of 'trio').
'almost' means to remove the last letter (most of the word but not all of it).
'quiet' becomes 'sh'.
'three' with its final letter taken away is 'thre'.
'thre'+'sh'='THRESH'
(Other definitions for thresh that I've seen before include "Extract grain from husks" , "Separate grain from (corn); move violently" , "Beat grain or make violent movements" , "Beat (stalks to get grain)" , "Separate grain from (eg, corn)" .)