Not in a medley race (6)
I believe the answer is:
nation
'race' is the definition.
(both can mean a national or ethnic group)
'not in a medley' is the wordplay.
'medley' is an anagram indicator.
'not'+'in'+'a'='notina'
'notina' with letters rearranged gives 'NATION'.
(Other definitions for nation that I've seen before include "The Birth of a -, D.W. Griffiths film" , "Country, state" , "American Indian people" , "People of common culture or descent" , "Anoint (anag)" .)