Crossword Genius

Ruin three-quarters of the planet (3)

Ross

I believe the answer is:

mar

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'ruin' is the definition.
(I've seen this before)

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

Can you help me to learn more?

(Other definitions for mar that I've seen before include "Spoil or deface" , "wreck" , "Damage; month (abbr.)" , "Make imperfect" , "Oh dear!" .)

I've seen this clue in the Evening Standard.
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