The most common and most likely answer to this crossword clue is the 7 letter word MUSTANG
I've seen this crossword clue in Financial Times
M
U
S
T
A
N
G
More clues leading to the result MUSTANG
- Greek character flies over wild horse
- Horse has to be killed? Horse escapes
- Wild horse has to enrage without hesitation
- Horse ‘as to be ‘eld up
- Horsefly problem is mounting
- Horse ‘as to be executed
- A few said to be upset by smell coming from horse
- Sucker riding small brown horse
- Fly problem upset horse
- Trotter, wild, should go to the gallows in Bow?
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 = mustang.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 MUSTANG mean?
MUSTANG noun- A small, hardy, naturalized (feral) horse of the North American west.
- A merchant marine who joined the U.S. Navy as a commissioned officer during the American Civil War.
- (generalized) A commissioned officer who started military service as an enlisted person.