The most common and most likely answer to this crossword clue is the 7 letter word EVENTER
I've seen this crossword clue in The Daily Telegraph
E
V
E
N
T
E
R
More clues leading to the result EVENTER
- Medical department that constantly admits horseman
- Horse has calm expression after losing its tail
- Level ground, with no rain, for Badminton competitor?
- Jumper always having hole in it?
- Flat terrain, not wet, for rider
- Competitive rider always covers slit in jacket
- Competitor always seizes opening
- Badminton competitor maybe, energy swelling
- Horseman always around hospital department
- Horseman maybe
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 = eventer.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 EVENTER mean?
EVENTER noun- A horse or type of horse used in the equestrian sport of eventing.
- (equestrian) The rider of a horse used in eventing.