The most common and most likely answer to this crossword clue is the 9 letter word YEARLINGS
I've seen this crossword clue in The Guardian
Y
E
A
R
L
I
N
G
S
Buddy explains!

The straight part
We know that "young horses" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case young horses = yearlings.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 YEARLINGS mean?
YEARLINGS 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.