Crossword Genius

Terrible tirade overwhelms me, not wanting a black mark (7)

Ross

I believe the answer is:

demerit

'black mark' is the definition.
(a black mark marks a demerit or fault)

I don't understand how the rest of the clue works.

(Other definitions for demerit that I've seen before include "Mark given for a fault" , "Something deserving censure" , "Conduct that deserves censure" , "Feature deserving censure" , "Fact deserving censure" .)

I've seen this clue in The Times.
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