A number quietly on edge (3)
I believe the answer is:
lip
'on edge' is the definition.
(I've seen this before)
'a number quietly' is the wordplay.
'a number' becomes 'li' (I can't explain this - if you can you should believe this answer much more).
'quietly' becomes 'p' (p for piano and quiet).
'li'+'p'='LIP'
Can you help me to learn more?
(Other definitions for lip that I've seen before include "sauciness" , "Rim; impudence (slang)" , "Edge; impudence" , "giving such insincere service" , "Top edge of a vessel" .)