Discarded actors start to object very loudly (4,3)
I believe the answer is:
cast off
'discarded' is the definition.
('cast-off' can be a synonym of 'discarded')
'actors start to object very loudly' is the wordplay.
'actors' becomes 'cast' (I've seen this before).
'start to' indicates taking the first letters.
'very loudly' becomes 'ff'.
The first letter of 'object' is 'o'.
'cast'+'o'+'ff'='CAST-OFF'
(Other definitions for cast off that I've seen before include "Abandon - remove mooring lines - knot stitches" , "When the stage has emptied" , "no longer required" , "Stop knitting - prepare for sea" , "abandoned" .)