Mark, the fellow with the muffler (4)
I believe the answer is:
scar
'mark' is the definition.
(synonyms)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for scar that I've seen before include "One left after having wound" , "Nasty mark" , "Remains of a wound" , "Steep cliff; skin mark" , "Mark left by the healing of injured tissue" .)