The most common and most likely answer to this crossword clue is the 6 letter word MUDDLE
I've seen this crossword clue in The Guardian
M
U
D
D
L
E
Buddy explains!

The straight part
We know that "Mix up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mix up = muddle.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 MUDDLE mean?
MUDDLE noun- A mixture; a confusion; a garble.
- A mixture of crushed ingredients, as prepared with a muddler.
- To mix together, to mix up; to confuse.
- To mash slightly for use in a cocktail.
- To dabble in mud.
- To make turbid or muddy.
- To think and act in a confused, aimless way.
- To cloud or stupefy; to render stupid with liquor; to intoxicate partially.
- To waste or misuse, as one does who is stupid or intoxicated.