The most common and most likely answer to this crossword clue is the 8 letter word MURMURED
I've seen this crossword clue in The Daily Telegraph
M
U
R
M
U
R
E
D
Buddy explains!
The straight part
We know that "grumbled" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case grumbled = murmured.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 MURMURED mean?
MURMURED verb- To grumble; to complain in a low, muttering voice, or express discontent at or against someone or something.
- To speak or make low, indistinguishable noise; to mumble, mutter.
- To say (something) indistinctly, to mutter.