The most common and most likely answer to this crossword clue is the 10 letter word CLYDESDALE
I've seen this crossword clue in The Guardian
C
L
Y
D
E
S
D
A
L
E
More clues leading to the result CLYDESDALE
Buddy explains!

The straight part
We know that "Horse" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case horse = clydesdale.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.