Sound bell for knockout? (9)
I believe the answer is:
humdinger
'knockout?' is the definition.
(I've seen this before)
'sound bell' is the wordplay.
'sound' becomes 'hum' (hum is a kind of sound).
'bell' becomes 'dinger' (I am not sure about this - if you are sure you should believe this answer much more).
'hum'+'dinger'='HUMDINGER'
'for' is the link.
Can you help me to learn more?
(Other definitions for humdinger that I've seen before include "One's brilliant" , "Something very large or excellent" , "something else" , "cracker" , "Peach" .)