Attorney left backing worm (5)

I believe the answer is:

lytta

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'worm' is the definition.
Both the answer and definition are singular nouns.
Perhaps you can see a link between them that I can't see?

I don't understand the rest of the clue.

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(Another definition for lytta that I've seen is " perceived source of rabies?".)

I've seen this clue in The Times and The Sunday Times.

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