Crossword Genius

Con artist enemy of Batman changing US party allegiance initially (7)

Ross

I believe the answer is:

diddler

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'con artist' is the definition.
(diddle can mean to con or defraud)

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for diddler that I've seen before include "Swindling person" , "Swindler" .)

I've seen this clue in The Independent.
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