The most common and most likely answer to this crossword clue is the 8 letter word YEARLING
I've seen this crossword clue in The Times
Y
E
A
R
L
I
N
G
More clues leading to the result YEARLING
- One from Glasgow hosts peer, good runner
- Time left at home with girl makes young horse
- Ludlow’s first hurdled by a grey, in labouring horse
- Young horse in yard needing attention shown by Heather
- In Argyle knit the makings of a jumper?
- You, old girl, with an unruly youngster
- Nigel Ray trained a young racehorse
- Elderly earl in gunroom holstering a Colt?
- Little jumper perhaps in Argyle knit?
- Heather, after twelve months, gets young animal
Buddy explains!

The straight part
We know that "runner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case runner = yearling.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does YEARLING mean?
YEARLING noun- An animal that is between one and two years old; one that is in its second year (but not yet two full years old).
- A racehorse that is considered to be one year old until a subsequent January 1st.
- A sophomore at West Point military academy.