Race with horses throughout the land (8)
I believe the answer is:
national
'race' is the definition.
(horse race the Grand National)
'with horses throughout the land' is the wordplay.
'with' means one lot of letters goes inside another.
'horses throughout' becomes 'ion' (I can't explain this - if you can you should give a lot more credence to this answer).
'the land' becomes 'Natal' (Natal is an example).
'ion' going inside 'natal' is 'NATIONAL'.
Can you help me to learn more?
(Other definitions for national that I've seen before include "Owned by the state" , "Relating to a country" , "A resident" , "See 17" , "It's a Grand race at Aintree" .)