The most common and most likely answer to this crossword clue is the 4 plus 2 plus 3 plus 4 letter word WHAT DO YOU KNOW
I've seen this crossword clue in The Daily Telegraph
W
H
A
T
D
O
Y
O
U
K
N
O
W
Buddy explains!
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.