One sod of course forgetting verse in Dumbo (5)

I believe the answer is:

idiot

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'one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for idiot that I've seen before include "Dope" , "Foolish character" , "Clot" , "one needing head examined?" , "Charlie" .)

I've seen this clue in The Guardian.

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