Crossword Genius

Sound bell for knockout? (9)


I believe the answer is:


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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).

'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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