The most common and most likely answer to this crossword clue is the 8 letter word CHUNTERS
C
H
U
N
T
E
R
S
Buddy explains!
The straight part
We know that "Grumbles" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case grumbles = chunters.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 CHUNTERS mean?
CHUNTERS verb- To speak in a soft, indistinct manner, mutter.
- To grumble, complain.