The most common and most likely answer to this crossword clue is the 10 letter word DRIVELLING
I've seen this crossword clue in The Daily Telegraph
D
R
I
V
E
L
L
I
N
G
Buddy explains!
The straight part
We know that "gibbering" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gibbering = drivelling.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 DRIVELLING mean?
DRIVELLING verb- To have saliva drip from the mouth; to drool.
- To talk nonsense; to talk senselessly; to drool.
- To be weak or foolish; to dote.
- Foolish talk.