The most common and most likely answer to this crossword clue is the 6 letter word DEVISE
I've seen this crossword clue in Independent
D
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More clues leading to the result DEVISE
Buddy explains!

The straight part
We know that "Plan" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plan = devise.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 DEVISE mean?
DEVISE noun- The act of leaving real property in a will.
- Such a will, or a clause in such a will.
- The real property left in such a will.
- Design, devising.
- To use one's intellect to plan or design (something).
- To leave (property) in a will.
- To form a scheme; to lay a plan; to contrive; to consider.
- To plan or scheme for; to plot to obtain.
- To imagine; to guess.