The most common and most likely answer to this crossword clue is the 4 plus 8 letter word LONG DIVISION
I've seen this crossword clue in The Sunday Telegraph
L
O
N
G
D
I
V
I
S
I
O
N
More clues leading to the result LONG DIVISION
- Method of calculating the Great Wall of China, say?
- Arithmetic I do in solving puzzle
- Faculty backs learner doing badly in mathematical process
- Far-sightedness about diamonds I included in share calculation
- Extensive calculating needed to create pine border
- Not a short class in arithmetic
- Protracted disunity that was once more common in primary schools
Buddy explains!
The straight part
We know that "share calculation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case share calculation = long 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.