Cryptic crossword clue
Fool, regarding single mistake, shut up 6,4,3

The most common and most likely answer to this crossword clue is the 6 plus 4 plus 3 letter word BUTTON ONES LIP

I've seen this crossword clue in The Sunday Telegraph
B
U
T
T
O
N
O
N
E
S
L
I
P

Buddy explains!

The straight part
We know that "shut up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case shut up = button ones lip.
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.

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