The most common and most likely answer to this crossword clue is the 7 plus 7 letter word CRYSTAL LATTICE
C
R
Y
S
T
A
L
L
A
T
T
I
C
E
Buddy explains!
The straight part
We know that "3D arrangement inside some solids" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case 3d arrangement inside some solids = crystal lattice.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.