The most common and most likely answer to this crossword clue is the 8 plus 7 letter word BINOMIAL THEOREM
I've seen this crossword clue in The Times
B
I
N
O
M
I
A
L
T
H
E
O
R
E
M
Buddy explains!
The straight part
We know that "a piece of mathematics" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a piece of mathematics = binomial theorem.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.