Mark made by unfinished muffler (4)
I believe the answer is:
scar
'mark' is the definition.
(mark left by an injury)
'unfinished muffler' is the wordplay.
'unfinished' means to remove the last letter.
'muffler' becomes 'scarf' (I've seen this before).
'scarf' with its last letter removed is 'SCAR'.
'made by' acts as a link.
(Other definitions for scar that I've seen before include "Evidence of a past injury?" , "Remains of a wound" , "Permanently damage" , "Visible mark of healing" , "Steep rock - blemish" .)