The most common and most likely answer to this crossword clue is the 8 letter word TINKERED
I've seen this crossword clue in Financial Times
T
I
N
K
E
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D
Buddy explains!

The straight part
We know that "messed around" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case messed around = tinkered.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 TINKERED mean?
TINKERED verb- To fiddle with something in an attempt to fix, mend or improve it, especially in an experimental or unskilled manner.
- To work as a tinker.
- To tinker with; to tweak or attempt to fix.