It decays a cad (6)

I believe the answer is:

rotter

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

I cannot understand how the remainder of the clue works.

(Other definitions for rotter that I've seen before include "Mean and unkind person" , "rat" , "Cruel and mean person" , "one going bad?" , "Bad lot" .)

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

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