The most common and most likely answer to this crossword clue is the 9 letter word LANTHANUM
L
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The straight part
We know that "La" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case la = lanthanum.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 LANTHANUM mean?
LANTHANUM noun- A chemical element (symbol La) with an atomic number of 57: a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air.
- An atom of this element.