Muscle shown by East End bully punching friend (8)
I believe the answer is:
pectoral
'muscle' is the definition.
(pectoral muscle is a kind of muscle)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for pectoral that I've seen before include "Relating to the breast or chest" , "Alert cop about chest" , "(Large muscle) of the chest" , "Of the chest, say muscles" , "Of the chest or breast, like muscles" .)