The most common and most likely answer to this crossword clue is the 8 plus 5 letter word PERIODIC TABLE
I've seen this crossword clue in The Times
P
E
R
I
O
D
I
C
T
A
B
L
E
More clues leading to the result PERIODIC TABLE
- List that starts at H
- He is second on it
- Unsound biopic altered elementary layout
- Occasional pharmacist finally accomplished chemist’s work
- He is second on this list
- He and I get into such rows!
- Stop one guy importing first of things for elementary arrangement
- What’s 1 across about me changing rhenium to erbium?
- He appears in this chart at number 2
- MThis arrangement establishes that copper is ordered not to follow lead!
Buddy explains!
The straight part
We know that "elementary layout" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case elementary layout = periodic table.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.