The most common and most likely answer to this crossword clue is the 11 letter word MATHEMATICS
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Buddy explains!
The straight part
We know that "It’s used to sum up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s used to sum up = mathematics.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 MATHEMATICS mean?
MATHEMATICS noun- An abstract representational system used in the study of numbers, shapes, structure, change and the relationships between these concepts.
- A person's ability to count, calculate, and use different systems of mathematics at differing levels.