The most common and most likely answer to this crossword clue is the 10 letter word SCHEMATICS
I've seen this crossword clue in The Guardian
S
C
H
E
M
A
T
I
C
S
More clues leading to the result SCHEMATICS
Buddy explains!

The straight part
We know that "Plans" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plans = schematics.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 SCHEMATICS mean?
SCHEMATICS noun- A simplified line drawing used by scientists, engineers, technologists and others to illustrate a system at an abstract level. Schematic drawings often use standard symbols for clarity.