1/9 across
Scientific
fact
maybe
lacking
substance
–
flat-earthers
most
stupid
(7,4,7)
Scientific fact maybe lacking substance – flat-earthers most stupid (7,4,7)
hints
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checks
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hints
The definition is at the beginning of the clue.
How the clue works
The definition is Scientific fact (loosely — it's really the famous maths theorem). First take 'maybe' lacking its substance (drop the middle letters) to leave ME, then add all the letters of 'flat-earthers most'. Jumbling that lot together, with 'stupid' as the anagram indicator, gives FERMATS LAST THEOREM.