Fish on strike? I don’t believe that (10)

I believe the answer is:

codswallop

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'i don't believe that' is the definition.
The definition suggests a singular noun which matches the answer.

'fish on strike?' is the wordplay.
'fish on' becomes 'cods' (I've seen this in another clue. I am not sure about the 'on' bit.).
'strike?' becomes 'wallop' (I've seen this before).
'cods'+'wallop'='CODSWALLOP'

Can you help me to learn more?

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

I've seen this clue in The Everyman puzzle in The Observer.

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