Biblical sibling sounds competent (4)

I believe the answer is:

abel

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'biblical sibling' is the definition.
The definition suggests a singular noun which matches the answer.

I don't understand the remainder of the clue.

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(Other definitions for abel that I've seen before include "The Third Man" , "Cain slew him" , "First person killed" , "done in before anyone else" , "Name" .)

I've seen this clue in The Daily Express.

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