The most common and most likely answer to this crossword clue is the 8 letter word WISEACRE
I've seen this crossword clue in The Times
W
I
S
E
A
C
R
E
More clues leading to the result WISEACRE
- Professor of knowledge we care about is accepted
- Sage's old communication covering main chapter
- Cable involving blueknow-all
- For what reason is a scoundrel said to be a know-all?
- Know-all I found on ocean in ragtag crew
- Know-all from Wisconsin, the main source of controversy over scripture
- Telegram about marine initially confounded smart alec
- Comedian right to probe outstanding know-all
- Smart Aleck’s telegram about resolved case
- Know-it-all spoils race after you and I take first and second
Buddy explains!

The straight part
We know that "know-all" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case know-all = wiseacre.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 WISEACRE mean?
WISEACRE noun- One who feigns knowledge or cleverness; one who is wisecracking; an insolent upstart.
- A learned or wise man.