Nation in competition (4)
I believe the answer is:
race
This clue is a double definition.
'nation' is the first definition.
(I've seen this before)
'competition' is the second definition.
(I know that race is a Race)
'in' is the link.
(Other definitions for race that I've seen before include "People; run" , "put their skates on" , "Run, dash" , "Speed competition" , "Move fast" .)