Customised leather rafts sail over the edge, if they're right (4-8)

I believe the answer is:

flat-earthers

'sail over the edge if they're right' is the definition.
(flat earthers believe Earth has edges)

'customised leather rafts' is the wordplay.
'leather rafts' can be anagrammed to 'FLAT-EARTHERS'.
Though, I am unsure how the anagram is indicated.

I've seen this clue in The Guardian.

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