The most common and most likely answer to this crossword clue is the 6 plus 6 letter word DANISH PASTRY
I've seen this crossword clue in The Times
D
A
N
I
S
H
P
A
S
T
R
Y
More clues leading to the result DANISH PASTRY
Buddy explains!

The straight part
We know that "quite a mouthful" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case quite a mouthful = danish pastry.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.