Nations in the running (5)
I believe the answer is:
races
'nations' is the definition.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for races that I've seen before include "Competitions; ethnic groups" , "Scare in sporting contests" , "Events at Epsom or Ascot?" , "Speed competitions for various peoples" , "Moves very fast" .)