The most common and most likely answer to this crossword clue is the 2 plus 5 plus 3 plus 6 letter word AT SIXES AND SEVENS
I've seen this crossword clue in The Times
A
T
S
I
X
E
S
A
N
D
S
E
V
E
N
S
More clues leading to the result AT SIXES AND SEVENS
Buddy explains!

The straight part
We know that "in a mess" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in a mess = at sixes and sevens.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.