The most common and most likely answer to this crossword clue is the 4 plus 6 letter word DEAD SECRET
I've seen this crossword clue in Financial Times
D
E
A
D
S
E
C
R
E
T
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.