The most common and most likely answer to this crossword clue is the 3 plus 2 letter word TOT UP
I've seen this crossword clue in The Sunday Telegraph
T
O
T
U
P
More clues leading to the result TOT UP
- Sum to put back
- Calculate the full value of babe in arms?
- Add set of books to insert into elevated place
- child out of bed? Put two and two together!
- Add final stroke, set up with nothing in it
- Sum to deposit, rising
- Reckon on endless sheep appearing after a short time
- Small child raised to do simple arithmetic
- Infant excited to perform addition
- Add drink at college
Buddy explains!

The straight part
We know that "Calculate" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case calculate = tot up.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.