They are not generally to be heard speaking (6)
I believe the answer is:
asides
'they' is the definition.
Both the answer and definition are plural nouns.
Perhaps you can see a link between them that I don't see?
I can't explain the remainder of the clue.
(Other definitions for asides that I've seen before include "They're not heard by everybody" , "Actors' remarks in an undertone" , "Stage whisper" , "Remarks to an audience" , "Stage whispers" .)