Note about keeping quiet is revealed (5)
I believe the answer is:
blown
'is revealed' is the definition.
'note about keeping quiet' is the wordplay.
'note' becomes 'nb' (**).
'about' says the letters should be written backwards.
'keeping' means one lot of letters goes inside another.
'quiet' becomes 'low'.
'nb' in reverse letter order is 'bn'.
'bn' placed around 'low' is 'BLOWN'.
(Other definitions for blown that I've seen before include "squandered" , "Whistle should be" , "Like some glass vessels" , "Moved by the wind" , "(Rose) in blossom; (cover) exposed" .)