The most common and most likely answer to this crossword clue is the 8 letter word GEOMETER
I've seen this crossword clue in The Sunday Telegraph
G
E
O
M
E
T
E
R
More clues leading to the result GEOMETER
Buddy explains!

The straight part
We know that "Mathematician" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mathematician = geometer.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 GEOMETER mean?
GEOMETER noun- A mathematician who specializes in geometry.
- Any species of geometrid moth (family Geometridae).