An old-fashined rotter (7)

I believe the answer is:

dastard

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

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

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(Other definitions for dastard that I've seen before include "Coward" , "Cowardly fellow" , "despicable fellow!" , "contemptible scaredy-cat" .)

I've seen this clue in the Evening Standard.

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