The most common and most likely answer to this crossword clue is the 9 letter word METHADONE
I've seen this crossword clue in The Times
M
E
T
H
A
D
O
N
E
Buddy explains!
The straight part
We know that "It might help one to get off horse" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it might help one to get off horse = methadone.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 METHADONE mean?
METHADONE noun- A synthetic opioid analgesic, used to wean addicts off heroin or other opiate based narcotics, and in chronic pain management.