The most common and most likely answer to this crossword clue is the 8 letter word DIVISION
I've seen this crossword clue in The Daily Telegraph
D
I
V
I
S
I
O
N
More clues leading to the result DIVISION
- Defence Intelligence seeing army unit
- Class’s / maths exercise, long or short?
- Possibly long calculation for army unit
- Dream girl heads section
- Duke, one with ability to see discord
- Papers over dream separation
- Inspector with dream department
- Parliamentary vote for a large military formation
- Boundary I had drawn up, ideal
- Mathematical process in class
Buddy explains!

The straight part
We know that "Mathematical process/class" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mathematical process/class = division.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 DIVISION mean?
DIVISION noun- The act or process of dividing anything.
- Each of the separate parts of something resulting from division.
- The process of dividing a number by another.
- A calculation that involves this process.
- A formation, usually made up of two or three brigades.
- A usually high-level section of a large company or conglomerate.
- A rank below kingdom and above class, particularly used of plants or fungi, also (particularly of animals) called a phylum; a taxon at that rank.
- A disagreement; a difference of viewpoint between two sides of an argument.
- A method by which a legislature is separated into groups in order to take a better estimate of vote than a voice vote.
- A florid instrumental variation of a melody in the 17th and 18th centuries, originally conceived as the dividing of each of a succession of long notes into several short ones.
- A set of pipes in a pipe organ which are independently controlled and supplied.
- A concept whereby a common group of debtors are only responsible for their proportionate sum of the total debt.
- Any of the four major parts of a COBOL program source code
- (Eton College) A lesson; a class.