The most common and most likely answer to this crossword clue is the 7 letter word BUNGLED
I've seen this crossword clue in Financial Times
B
U
N
G
L
E
D
More clues leading to the result BUNGLED
Buddy explains!

The straight part
We know that "Made a complete mess" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case made a complete mess = bungled.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.
What does BUNGLED mean?
BUNGLED verb- To botch up, bumble or incompetently perform a task; to make or mend clumsily; to manage awkwardly.