For a mare to get old will do no good at all (6)
I believe the answer is:
damage
'for' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see a link between them that I can't see?
I cannot 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)'" .)