Crossword Genius

What decays a cad? (6)

Ross

I believe the answer is:

rotter

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'cad?' is the definition.
(I've seen this before)

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

Can you help me to learn more?

(Other definitions for rotter that I've seen before include "Pig" , "Despicable person" , "baddie" , "rat" , "one going bad?" .)

I've seen this clue in the Sydney Morning Herald.
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