The most common and most likely answer to this crossword clue is the 8 letter word MULTIPLY
M
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Buddy explains!

The straight part
We know that "breed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case breed = multiply.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 MULTIPLY mean?
MULTIPLY noun- An act or instance of multiplying.
- To increase the amount, degree or number of (something).
- (with by) To perform multiplication on (a number).
- To grow in number.
- To breed or propagate.
- To perform multiplication.
- To be a factor in a multiplication with (another factor).
- In many or multiple ways.