Trio cut short quiet beat (6)
I believe the answer is:
thresh
'beat' is the definition.
(threshing is a kind of beating)
'trio cut short quiet' is the wordplay.
'trio' becomes 'three' ('three' can be a synonym of 'trio').
'cut short' means to remove the last letter.
'quiet' becomes 'sh'.
'three' with its last letter taken off is 'thre'.
'thre'+'sh'='THRESH'
(Other definitions for thresh that I've seen before include "Separate grain from husks and straw" , "Use flail (on grain crop)" , "Beat grain or make violent movements" , "Separate grain from (corn); move violently" , "Beat (stalks to get grain)" .)