Cad going from bad to worse (6)

I believe the answer is:

rotter

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

'bad to worse' is the wordplay.
'bad' becomes 'rot' (I've seen this before).
'to worse' becomes 'ter' (I can't explain this - if you can you should give a lot more credence to this answer).
'rot'+'ter'='ROTTER'

'going from' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.

Can you help me to learn more?

(Other definitions for rotter that I've seen before include "rat" , "Rogue" , "Bad guy" , "Absolute bounder" , "Pig" .)

I've seen this clue in the Evening Standard.

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