Crossword Genius

Ghastly smut, mostly (4)

Ross

I believe the answer is:

grim

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'ghastly' is the definition.
(I know that ghastly can be written as grim)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for grim that I've seen before include "Severe; horrible" , "Resolute" , "Unrelentingly harsh" , "Serious; dour" , "Dire" .)

I've seen this clue in The Independent.
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