4 letters:
* ABSCOND (to leave secretly and hurriedly, especially to escape from custody)
5 letters:
* BOLT (to run away suddenly and quickly)
* FLEE (to run away from danger or an unpleasant situation)
6 letters:
* SCRAM (to leave hurriedly or unceremoniously)
* DEPART (to leave a place, especially at the end of a journey or stay)
7 letters:
* HIGHTAIL (to leave quickly and hastily)
* SKEDADDLE (to run away quickly, especially because of fear)
8 letters:
* MAKE_OFF (to leave hurriedly, especially with something that has been stolen)
To find the best fit for your puzzle, consider the following:
* The length of the blank space.
* The surrounding words and their themes.
* The overall tone of the puzzle.
Good luck with your crossword!