The most common and most likely answer to this crossword clue is the 9 letter word HORSEBACK
I've seen this crossword clue in Financial Times
H
O
R
S
E
B
A
C
K
More clues leading to the result HORSEBACK
Buddy explains!
The straight part
We know that "riding on this?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case riding on this? = horseback.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 HORSEBACK mean?
HORSEBACK noun- (usually with on, upon, etc.) The back of a horse.
- A ridge of sand, gravel, and boulders.
- On the back of a horse.