The most common and most likely answer to this crossword clue is the 8 letter word KNOW-ALL
I've seen this crossword clue in The Guardian
K
N
O
W
-
A
L
L
Buddy explains!
The straight part
We know that "wise guy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case wise guy = know-all.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.
Crossword Answer definitions
KNOW-ALL noun- Someone who obnoxiously claims to be knowledgeable on a subject.