The most common and most likely answer to this crossword clue is the 14 letter word DILLY-DALLYING
I've seen this crossword clue in The Guardian
D
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Buddy explains!
The straight part
We know that "just messing about!/ just messing about!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case just messing about!/ just messing about! = dilly-dallying.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 DILLY-DALLYING mean?
DILLY-DALLYING verb- To dawdle; to waste time; to procrastinate.