It's useful when bending the elbow (6)

I believe the answer is:

biceps

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'elbow' is the definition.
(thesaurus)

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

(Other definitions for biceps that I've seen before include "Muscle that flexes the forearm" , "Arm muscles" , "Weightlifters use them" , "Arm workers" , "Elbow-bending muscle" .)

I've seen this clue in the Evening Standard.

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