Any person can claim to be unique! (3)
I believe the answer is:
one
This clue is a quadruple definition!!
'any' is the first definition.
(thesaurus)
'person' is the second definition.
('one' can mean an unspecified person)
'can claim' is the third definition.
The answer and definition can be both quantities as well as being singular nouns.
Maybe there's an association between them I don't understand?
'to' acts as a link.
'be unique' is the fourth definition.
(synonyms)
Can you help me to learn more?
(Other definitions for one that I've seen before include "Leading cardinal" , "United" , "doesn't count for much" , "Smallest whole number" , "Unit" .)