Harness racer (7)
I believe the answer is:
trotter
'harness racer' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for trotter that I've seen before include "horse getting sulky, say" , "Pig's foot, or one on the run" , "Steady horse; pig's foot" , "Briskly moving horse - pig's foot" , "foot of animal for food" .)