Rubbish swimmer's huge impact (10)

I believe the answer is:

codswallop

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'rubbish' is the definition.
(I know that rubbish can be written as codswallop)

'swimmer's huge impact' is the wordplay.
'swimmer' becomes 'cods' (I've seen this in another clue).
'huge impact' becomes 'wallop' (I can't justify this - if you can you should believe this answer much more).
'cods'+'wallop'='CODSWALLOP'

Can you help me to learn more?

(Other definitions for codswallop that I've seen before include "[BUNKUM]" , "tripe" , "waffle?" , "Hogwash" , "Baloney" .)

I've seen this clue in The Telegraph.

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