The most common and most likely answer to this crossword clue is the 8 letter word SPLUTTER
I've seen this crossword clue in Financial Times
S
P
L
U
T
T
E
R
Buddy explains!

The straight part
We know that "talk incoherently" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case talk incoherently = splutter.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 SPLUTTER mean?
SPLUTTER noun- A spluttering.
- To sputter.
- To spray droplets of saliva from the mouth while speaking.
- To speak hurriedly and confusedly.
- To perform to a substandard level.