The most common and most likely answer to this crossword clue is the 9 plus 5 plus 8 letter word IRRITABLE BOWEL SYNDROME
I've seen this crossword clue in The Guardian
I
R
R
I
T
A
B
L
E
B
O
W
E
L
S
Y
N
D
R
O
M
E
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.