In this car is a nasty mark (4)
I believe the answer is:
scar
'a nasty mark' is the definition.
(I've seen this before)
'in this car is' is the wordplay.
'in' indicates a hidden word.
'SCAR' is hidden in the letters of 'this car is'.
(Other definitions for scar that I've seen before include "A bare precipice" , "area of wound" , "Cicatrice" , "Mark of old injury" , "Mark left by old wound" .)