Discard actors' whisper (4,5)
I believe the answer is:
cast aside
'discard' is the definition.
(synonyms)
'actors' whisper' is the wordplay.
'actors'' becomes 'cast' (group of actors).
'whisper' becomes 'aside'.
'cast'+'aside'='CAST ASIDE'
(Other definitions for cast aside that I've seen before include "dropped" , "Get rid of" , "Ditch" .)