The most common and most likely answer to this crossword clue is the 12 letter word TRIGONOMETRY
I've seen this crossword clue in Financial Times
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More clues leading to the result TRIGONOMETRY
Buddy explains!
The straight part
We know that "describing branch of mathematics" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case describing branch of mathematics = trigonometry.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 TRIGONOMETRY mean?
TRIGONOMETRY noun- The branch of mathematics that deals with the relationships between the sides and the angles of triangles and the calculations based on them, particularly the trigonometric functions.