The most common and most likely answer to this crossword clue is the 7 letter word FISTULA
I've seen this crossword clue in Financial Times
F
I
S
T
U
L
A
More clues leading to the result FISTULA
Buddy explains!

The straight part
We know that "problem for horse" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case problem for horse = fistula.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 FISTULA mean?
FISTULA noun- An abnormal connection or passageway between organs or vessels that normally do not connect.
- A tube, a pipe, or a hole.
- The tube through which the wine of the Eucharist was once sucked from the chalice.