The most common and most likely answer to this crossword clue is the 15 letter word UNACCOUNTED-FOR
I've seen this crossword clue in The Times
U
N
A
C
C
O
U
N
T
E
D
-
F
O
R
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.