Early leaver (3)

I believe the answer is:

rat

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'early leaver?' is the definition.
The definition and answer can be both people as well as being singular nouns.
Maybe you can see an association between them that I can't see?

This is the entire clue.

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(Other definitions for rat that I've seen before include "Rodent - deserter" , "Despicable person" , "Plague-carrying rodent" , "Destructive rodent" , "shopper" .)

I've seen this clue in the Evening Standard and The Globe and Mail.

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