The most common and most likely answer to this crossword clue is the 11 letter word QUICKSILVER
I've seen this crossword clue in The Times
Q
U
I
C
K
S
I
L
V
E
R
Buddy explains!

The straight part
We know that "Element" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case element = quicksilver.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 QUICKSILVER mean?
QUICKSILVER noun- The metal mercury.
- An amalgam of mercury and tin applied to the backs of mirrors, quicksilvering.
- To overlay with quicksilver.
- To treat with quicksilver.
- Unpredictable, erratic or fickle; mercurial.