US party political switch in social trick (9)

I believe the answer is:

deception

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'trick' is the definition.
(I know that trick can be written as deception)

I cannot understand how the rest of the clue works.

(Other definitions for deception that I've seen before include "Trick" , "Duplicity" , "Fraud" , "Illusion" .)

I've seen this clue in The Guardian.

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