The most common and most likely answer to this crossword clue is the 9 letter word GROUCHIER
I've seen this crossword clue in Independent
G
R
O
U
C
H
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E
R
Buddy explains!

The straight part
We know that "more grumbling" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case more grumbling = grouchier.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 GROUCHIER mean?
GROUCHIER adjective- (originally student slang) Irritable; easily upset; angry; tending to complain.