The most common and most likely answer to this crossword clue is the 8 letter word SPAWNING
I've seen this crossword clue in The Times
S
P
A
W
N
I
N
G
More clues leading to the result SPAWNING
Buddy explains!

The straight part
We know that "breeding" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case breeding = spawning.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 SPAWNING mean?
SPAWNING verb- To produce or deposit (eggs) in water.
- To generate, bring into being, especially non-mammalian beings in very large numbers.
- To bring forth in general.
- To induce (aquatic organisms) to spawn.
- To plant with fungal spawn.
- To deposit (numerous) eggs in water.
- To reproduce, especially in large numbers.
- (of a character or object) (To cause) to appear spontaneously in a game at a certain point and time.
- The process by which something spawns.