A ready rhyme for an ass (5)

I believe the answer is:

neddy

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'for an ass' is the definition.
(thesaurus)

I don't understand how the remainder of the clue works.

(Other definitions for neddy that I've seen before include "Donkey (colloq.)" , "One of child's 23 ac" .)

I've seen this clue in the Evening Standard.

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