The most common and most likely answer to this crossword clue is the 9 letter word BATTLEBUS
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Buddy explains!
The straight part
We know that "vehicle for campaigning Brexiteers?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case vehicle for campaigning brexiteers? = battlebus.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 BATTLEBUS mean?
BATTLEBUS noun- A coach used as a mobile operational centre by a particular political party during an election campaign.
- A bus or coach used as a personnel carrier for combatants to deliver them to the battlefield and then rolled on its side to provide cover against firearms.