What a dog may do, the rotter! (4)

I believe the answer is:

heel

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'the rotter' is the definition.
(I've seen this in another clue)

I don't understand the remainder of the clue.

(Other definitions for heel that I've seen before include "A cad and a bounder!" , "Part of shoe or informal scoundrel" , "Scoundrel - list - shoe part" , "Spur" , "command given to setter, say" .)

I've seen this clue in the Evening Standard.

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