The most common and most likely answer to this crossword clue is the 5 plus 9 letter word DOG’S BREAKFAST
D
O
G
â
€
™
S
B
R
E
A
K
F
A
S
T
Buddy explains!

The straight part
We know that "A complete shambles" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a complete shambles = dog’s breakfast.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.