Explain losing power for traitor (7)

I believe the answer is:

sellout

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'traitor' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Another definition for sellout that I've seen is " Commercial success - total betrayal".)

I've seen this clue in The Irish Times.

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