Cake for an old mule (6)

I believe the answer is:

muffin

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'cake' is the definition.
(I know that muffin is a type of cake)

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

(Other definitions for muffin that I've seen before include "Round bread-like cake" , "Type of cake" , "Tea cake" , "Round raised cake, often split and toasted" , "Toasted, spongy bread roll" .)

I've seen this clue in The Daily Express.

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