Muscle, bit by ear affecting the mouth (8)
I believe the answer is:
pectoral
'muscle' is the definition.
(I know that pectoral is a type of striated muscle tissue)
I cannot understand how the remainder of the clue works.
(Other definitions for pectoral that I've seen before include "Ornamental breastplate" , "Of the chest or breast, like muscles" , "Of the chest, say muscles" , "Alert cop about chest" , "sort of cross" .)