The most common and most likely answer to this crossword clue is the 4 plus 3 plus 6 plus 3 letter word WHAT THE BUTLER SAW
I've seen this crossword clue in The Guardian
W
H
A
T
T
H
E
B
U
T
L
E
R
S
A
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.