The most common and most likely answer to this crossword clue is the 8 letter word THALLIUM
I've seen this crossword clue in The Guardian
T
H
A
L
L
I
U
M
More clues leading to the result THALLIUM
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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 THALLIUM mean?
THALLIUM noun- A metallic chemical element (symbol Tl) with atomic number 81: a gray post-transition metal that discolors when exposed to air.
- A single atom of this element.