Long race (8)
I believe the answer is:
marathon
'long race' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for marathon that I've seen before include "Race of about 26 miles" , "Roman hat (anag) - race" , "26-mile foot race" , "The longest foot-race" , "Long run (with a wall to get through!)" .)