Elbow benders (6)
I believe the answer is:
biceps
'elbow' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for biceps that I've seen before include "Large upper arm muscle" , "Arm muscles" , "I bend the elbow" , "Elbow-bending muscle" , "One operating arm" .)