The most common and most likely answer to this crossword clue is the 13 letter word INTESTINALFLU
I
N
T
E
S
T
I
N
A
L
F
L
U
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.