Wrongly retort he is a cad (6)

I believe the answer is:

rotter

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

'wrongly retort he' is the wordplay.
'retort' anagrams to 'ROTTER'.
But, I'm not sure how this is indicated.

Can you help me to learn more?

(Other definitions for rotter that I've seen before include "An informal cad, may be decaying" , "One not to be honoured" , "Scoundrel" , "rat" , "one going bad?" .)

I've seen this clue in The Globe and Mail.

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