Twice accept mushroom using muscle (6)
I believe the answer is:
biceps
'muscle' is the definition.
(biceps is a kind of muscle)
'twice accept mushroom' is the wordplay.
'twice' becomes 'bis' (I've seen this in other clues).
'accept' is an insertion indicator.
'mushroom' becomes 'cep' (Boletus edulis - an edible mushroom).
'bis' enclosing 'cep' is 'BICEPS'.
'using' is the link.
(Other definitions for biceps that I've seen before include "One operating arm" , "Large upper arm muscle" , "Elbow-bending muscle" , "Muscles on upper arms" , "Arm workers" .)