The most common and most likely answer to this crossword clue is the 5 letter word SURFS
I've seen this crossword clue in The Daily Telegraph
S
U
R
F
S
Buddy explains!

The straight part
We know that "Rides a white horse? (" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rides a white horse? ( = surfs.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 SURFS mean?
SURFS noun- Waves that break on an ocean shoreline.
- An instance or session of riding a surfboard in the surf.
- The bottom of a drain.
- To ride a wave, usually on a surfboard.
- To browse the Internet, television, etc.