In general, black marks (5)
I believe the answer is:
smuts
'black marks' is the definition.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for smuts that I've seen before include "South African field marshal and premier" , "signs of pollution" , "Specks of soot - South African statesman, d. 1950" , "In South Africa general" .)