Biceps or triceps (6)
I believe the answer is:
muscle
I believe this clue is a double definition.
'biceps' is the first definition.
(thesaurus)
'triceps' is the second definition.
(thesaurus)
(Other definitions for muscle that I've seen before include "Bicep or tricep, for instance" , "contractor" , "Physical power" , "Beef" , "Part of the body which produces movement" .)