The most common and most likely answer to this crossword clue is the 4 letter word ARAB
I've seen this crossword clue in The Times
A
R
A
B
More clues leading to the result ARAB
- On return, exclude a horse
- A device that stops one backing horse
- Sailor framing artist’s horse
- A device that’d stop you backing horse
- Supporter heading west astride a horse
- Separable clothes horse
- Semite in Basra disheartened and sent back
- Rail, one ascending mount
- Save a rising Semite
- All but one mounted horse
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 = arab.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.