There's many a one in our race (3)
I believe the answer is:
man
'race' is the definition.
(the human race)
'there's many a one' is the wordplay.
'there's' means one lot of letters go next to another.
'many' becomes 'M' (Roman numeral for a thousand).
'a one' becomes 'an' (an apple is one apple).
'm'+'an' is 'MAN'.
'in our' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for man that I've seen before include "race" , "just one" , "Homo sapiens" , "The most destructive and polluting animal" , "Provide with crew" .)