Young racehorse (8)
I believe the answer is:
yearling
'young racehorse' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for yearling that I've seen before include "Animal between 1 and 2 in age" , "Sort of racehorse" , "A racehorse under 2" , "Young horse" , "Relaying (anag.)" .)