The most common and most likely answer to this crossword clue is the 4 plus 4 letter word TURN TAIL
I've seen this crossword clue in The Times
T
U
R
N
T
A
I
L
More clues leading to the result TURN TAIL
- Prepare to run: go, dog!
- Go back and prepare to run
- Performer having time trouble in ‘Beat It‘?
- Spin yarn, say, and leave
- Reveal oneself to be a beastly coward
- Hear legend of seabird and run away
- Spin yarn, so to speak, and scarper
- Story heard after convert’s escape
- Go round dog and leg it
- Go off with dog and escape
Buddy explains!

The straight part
We know that "Prepare to run" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case prepare to run = turn tail.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.