The most common and most likely answer to this crossword clue is the 7 letter word CADMIUM
I've seen this crossword clue in The Guardian
C
A
D
M
I
U
M
More clues leading to the result CADMIUM
- Mother protects one following rogue element
- Metal heel: one secured by old lady
- IT-based design with not a word about one reactor element
- See promo silent about independent element
- Protected by relative, one goes after rogue element
- CD music made poorly, lacking special energy
- In copper money almost allow a sort of yellow
- Blackguard saying nothing about one metal
- Charlie and Bill silent about independent element
- 5d
Buddy explains!

The straight part
We know that "CD" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cd = cadmium.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 CADMIUM mean?
CADMIUM noun- A chemical element (symbol Cd) with an atomic number of 48: a soft, silvery-white metal.
- A single atom of this element.