Crossword Genius

Rubbish fish mostly gulp near edge of pond (10)

Ross

I believe the answer is:

codswallop

I'm a little stuck... Click here to teach me more about this clue!

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

'fish mostly gulp near edge of pond' is the wordplay.
I cannot quite understand how this works, but
'fish' could be 'cods' (I've seen this in another clue) and 'cods' is found within the answer.
'of' could be 'o' and 'o' is found within the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for codswallop that I've seen before include "Rubbish!" , "[BUNKUM]" , "tripe" , "Baloney" , "Nonsense (colloq.)" .)

I've seen this clue in The Independent.
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