By which meatballs made from stale lamb? (7)

I believe the answer is:

anagram

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'lamb?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

I can't explain the remainder of the clue.

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(Other definitions for anagram that I've seen before include "Shuffling of letters" , "Gelding can be niggled by this" , "Clue about a ragman?" , "A ragman (anag)" , "new order, literally" .)

I've seen this clue in The Sun.

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