Not a whisper to the audience! (5)
I believe the answer is:
aside
'not' is the definition.
Both the definition and answer are adverbs. Maybe you can see a link between them that I can't see?
'a whisper to the audience' is the wordplay.
'whisper to the audience' becomes 'side' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'side'='ASIDE'
Can you help me to learn more?
(Other definitions for aside that I've seen before include "Actor's remark for the audience alone" , "Quiet words spoken" , "Privately, apart" , "I.e. sad (anag.)" , "Actor's covert line" .)