The person declared to have won (3)
I believe the answer is:
one
'won' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Maybe there's a link between them I don't understand?
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for one that I've seen before include "Figure" , "doesn't count for much" , "Hour after midnight" , "If upper-class, perhaps I" , "This is singular" .)