The most common and most likely answer to this crossword clue is the 3 letter word MIX
I've seen this crossword clue in The Daily Telegraph
M
I
X
Buddy explains!

The straight part
We know that "mess" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mess = mix.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 MIX mean?
MIX verb- To stir together.
- To combine (items from two or more sources normally kept separate).
- To form by mingling; to produce by the stirring together of ingredients; to concoct from different parts.
- To blend by the use of a mixer (machine).
- To combine (several tracks).
- To produce a finished version of (a recording).
- To unite with in company; to join; to associate.
- The result of mixing two or more substances; a mixture.
- The result of combining items normally kept separate.
- The result of mixing several tracks.
- The finished version of a recording.