The most common and most likely answer to this crossword clue is the 10 letter word STABLEBOYS
I've seen this crossword clue in The Daily Telegraph
S
T
A
B
L
E
B
O
Y
S
More clues leading to the result STABLEBOYS
Buddy explains!

The straight part
We know that "hold your horses" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hold your horses = stableboys.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 STABLEBOYS mean?
STABLEBOYS noun- A boy or young man who attends in a stable.