The most common and most likely answer to this crossword clue is the 8 letter word UNSADDLE
I've seen this crossword clue in The Guardian
U
N
S
A
D
D
L
E
Buddy explains!
The straight part
We know that "to bring rider to earth" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to bring rider to earth = unsaddle.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 UNSADDLE mean?
UNSADDLE verb- To remove a saddle.
- To throw (a rider) from the saddle.