Former revolutionary's twin getting backside tweaked (6)

I believe the answer is:

castro

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'former revolutionary's twin' is the definition.
The definition suggests a singular noun which matches the answer.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for castro that I've seen before include "revolutionary leader" , "Man of Cuba" , "Old leader" , "Latin American dictator" , "Fidel ......, Cuban chief" .)

I've seen this clue in The Independent.

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