Saw three pigs cast aside (5,7)
I believe the answer is:
stage whisper
'aside' is the definition.
(I've seen this before)
'saw three pigs cast' is the wordplay.
'cast' indicates an anagram.
'saw'+'three'+'pigs'='sawthreepigs'
'sawthreepigs' with letters rearranged gives 'STAGE-WHISPER'.
(Other definitions for stage whisper that I've seen before include "something the audience should hear" , "Actor's aside" , "Utterance by an actor, supposedly not heard by the other players" , "Actor's quiet remark meant for the audience to hear" , "Ostensibly private aside intended to be heard by audience" .)