The most common and most likely answer to this crossword clue is the 4 letter word WHOA
I've seen this crossword clue in The Times
W
H
O
A
More clues leading to the result WHOA
- Stop sending up old husband and wife on April 1st
- What person has a command to stop?
- Doctor comes to a halt
- Stop suffering on the phone
- Stop TV doctor beginning to amputate
- From TV doctor, a directive to slow down
- Question a command from rider
- What to say to a horse while holding on anxiously, primarily?
- Call to stop lying in new hoax
- Wild horse overly accelerating initially – stop!
Buddy explains!

The straight part
We know that "Stop" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stop = whoa.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 WHOA mean?
WHOA verb- To attempt to slow (an animal) by crying "whoa".
- Stop (especially when commanding a horse or imitative thereof); calm down; slow down.
- An expression of surprise.
- Used as a meaningless filler in song lyrics.