The most common and most likely answer to this crossword clue is the 6 letter word BODGER
I've seen this crossword clue in The Guardian
B
O
D
G
E
R
More clues leading to the result BODGER
Buddy explains!

The straight part
We know that "One messes up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one messes up = bodger.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 BODGER mean?
BODGER noun- A woodworker in the traditional style characterised by the use of hand tools, a pole lathe and use of green timber.
- One who works in a rough and ready, slipshod manner.