Crossword Genius

Sound bell for knockout? (9)

Ross

I believe the answer is:

humdinger

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'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" .)

I've seen this clue in The Sun.
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