The most common and most likely answer to this crossword clue is the 11 letter word MENDELEVIUM
I've seen this crossword clue in The Times
M
E
N
D
E
L
E
V
I
U
M
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The straight part
We know that "Unstable element" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unstable element = mendelevium.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 MENDELEVIUM mean?
MENDELEVIUM noun- A transuranic chemical element (symbol Md, formerly Mv) with atomic number 101.