The most common and most likely answer to this crossword clue is the 7 letter word THORIUM
I've seen this crossword clue in Independent
T
H
O
R
I
U
M
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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 THORIUM mean?
THORIUM noun- A chemical element (symbol Th) with atomic number 90: a weakly radioactive, malleable, moderately hard silvery metal that tarnishes black when exposed to air.