The most common and most likely answer to this crossword clue is the 9 letter word SCHEDULER
I've seen this crossword clue in The Daily Telegraph
S
C
H
E
D
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L
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R
Buddy explains!
The straight part
We know that "planner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case planner = scheduler.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 SCHEDULER mean?
SCHEDULER noun- A person or device that determines a schedule, that determines the order that tasks are to be done.
- An operating system component responsible for allocating several resources, most commonly the use of processors by different concurrent processes or threads.