The most common and most likely answer to this crossword clue is the 8 letter word BABBLING
I've seen this crossword clue in The Guardian
B
A
B
B
L
I
N
G
Buddy explains!

The straight part
We know that "Incoherent" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case incoherent = babbling.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 BABBLING mean?
BABBLING verb- To utter words indistinctly or unintelligibly; to utter inarticulate sounds
- To talk incoherently; to utter meaningless words.
- To talk too much; to chatter; to prattle.
- To make a continuous murmuring noise, like shallow water running over stones.
- To utter in an indistinct or incoherent way; to repeat words or sounds in a childish way without understanding.
- To reveal; to give away (a secret).
- A stage in child language acquisition, during which an infant appears to be experimenting with uttering sounds of language, but not yet producing any recognizable words
- Sounds produced by infant during the babbling period
- Idle senseless talk; prattle.
- A confused murmur, as of a stream.