The most common and most likely answer to this crossword clue is the 7 plus 6 letter word BUBONIC PLAGUE
I've seen this crossword clue in Financial Times
B
U
B
O
N
I
C
P
L
A
G
U
E
More clues leading to the result BUBONIC PLAGUE
Buddy explains!

The straight part
We know that "terrible disorder" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case terrible disorder = bubonic plague.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.