The most common and most likely answer to this crossword clue is the 8 letter word MUMBLING
I've seen this crossword clue in Financial Times
M
U
M
B
L
I
N
G
More clues leading to the result MUMBLING
Buddy explains!

The straight part
We know that "Muttering" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case muttering = mumbling.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 MUMBLING mean?
MUMBLING verb- To speak unintelligibly or inaudibly; to fail to articulate.
- To chew something gently with closed lips.
- An act in which someone mumbles something