The most common and most likely answer to this crossword clue is the 9 letter word TRIHEDRAL
T
R
I
H
E
D
R
A
L
Buddy explains!
The straight part
We know that "having three planes" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case having three planes = trihedral.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 TRIHEDRAL mean?
TRIHEDRAL adjective- Having three plane faces that meet at a common point