Horse race across the country (8)
I believe the answer is:
national
'the country' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'horse race across' is the wordplay.
'horse' becomes 'al' (I can't explain this - if you can you should believe this answer much more).
'race' becomes 'nation' (both can mean a national or ethnic group).
'across' is a charade indicator (letters next to each other) (across can mean on).
'al' after 'nation' is 'NATIONAL'.
Can you help me to learn more?
(Other definitions for national that I've seen before include "Of the home country" , "State-owned" , "Major newspaper" , "Organism requiring free oxygen to live" , "Owned by the state" .)