It's used in bending the elbow (6)

I believe the answer is:

biceps

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'elbow' is the definition.
(thesaurus)

I don't understand the remainder of the clue.

(Other definitions for biceps that I've seen before include "Type of muscle" , "Muscle that flexes the forearm" , "One operating arm" , "Elbow-bending muscle" , "Upper-arm muscle" .)

I've seen this clue in the Evening Standard.

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