Race over fences (12)
I believe the answer is:
steeplechase
'race over fences' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for steeplechase that I've seen before include "Race over fences" , "Horse race with jumps and ditches" , "Running event over hurdles and water jumps" , "Type of horse race" , "Race with obstacles to be jumped" .)