The most common and most likely answer to this crossword clue is the 4 letter word OPAL
I've seen this crossword clue in The Times
O
P
A
L
More clues leading to the result OPAL
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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 OPAL mean?
OPAL noun- A mineral consisting, like quartz, of silica, but inferior to quartz in hardness and specific gravity, of the chemical formula SiO2·nH2O.
- A colloquial name used in molecular biology referring to a particular stop codon sequence, "UGA."
- Any of various lycaenid butterflies of the genus Nesolycaena.