The most common and most likely answer to this crossword clue is the 6 letter word FERMAT
I've seen this crossword clue in The Guardian
F
E
R
M
A
T
More clues leading to the result FERMAT
- 8‘s expression without opening round brackets
- Calculating sort, first to eat, jolly plump in girth
- Number theorem ends in stunt, as mathematician
- Mathematician‘s achievement displayed outside empty room
- Some duffer mathematically? Yes and no!
- French mathematician presents paper about a prime, oddly without using pi
Buddy explains!
The straight part
We know that "8" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case 8 = fermat.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.