The most common and most likely answer to this crossword clue is the 13 letter word FLAT-EARTHERS
I've seen this crossword clue in The Guardian
F
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Buddy explains!
The straight part
We know that "sail over the edge, if they’re right" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sail over the edge, if they’re right = flat-earthers.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 FLAT-EARTHERS mean?
FLAT-EARTHERS noun- A person who believes or advocates the theory that the planet Earth is flat.
- (by extension) A person who believes or advocates an outlandish, discredited theory; a person who refuses to acknowledge the truth despite overwhelming evidence.