The most common and most likely answer to this crossword clue is the 15 letter word DISORGANISATION
I've seen this crossword clue in The Daily Telegraph
D
I
S
O
R
G
A
N
I
S
A
T
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O
N
More clues leading to the result DISORGANISATION
Buddy explains!

The straight part
We know that "Lack of proper planning" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lack of proper planning = disorganisation.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 DISORGANISATION mean?
DISORGANISATION noun- The act of disorganizing; destruction of system.
- The state of being disorganized