Crossword Genius

For a mare to get old will do no good at all (6)

Ross

I believe the answer is:

damage

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'for' is the definition.
Both the definition and answer are singular nouns.
Maybe they are linked in a way I don't understand?

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

Can you help me to learn more?

(Other definitions for damage that I've seen before include "To vandalise" , "vandalism?" , "Deface, harm" , "Ravages" , "'Deface, impair (6)'" .)

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