The most common and most likely answer to this crossword clue is the 4 plus 5 plus 6 letter word RARE EARTH METALS
I've seen this crossword clue in The Times
R
A
R
E
E
A
R
T
H
M
E
T
A
L
S
Buddy explains!
The straight part
We know that "Certain elements" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case certain elements = rare earth metals.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.