The most common and most likely answer to this crossword clue is the 9 letter word FORECASTS
I've seen this crossword clue in The Guardian
F
O
R
E
C
A
S
T
S
Buddy explains!

The straight part
We know that "Prior calculations" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case prior calculations = forecasts.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 FORECASTS mean?
FORECASTS noun- An estimation of a future condition.
- (betting) exacta
- To estimate how something will be in the future.
- To foreshadow; to suggest something in advance.
- To contrive or plan beforehand.