The most common and most likely answer to this crossword clue is the 4 letter word FLEE
I've seen this crossword clue in The Times
F
L
E
E
More clues leading to the result FLEE
- Left in charge, head for the hills
- Run off with high jumper, you say?
- Run away from what may bite you, say
- Left in charge, do a runner
- Armada stopped early to run away
- Falterer on regular basis? Run away
- Bolt fast almost
- . Something biting you, you say? Run for it!
- Armada having no time to escape
- Leg it with circus performer, did you say?
Buddy explains!

The straight part
We know that "do a runner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case do a runner = flee.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 FLEE mean?
FLEE verb- To run away; to escape.
- To escape from.
- To disappear quickly; to vanish.