The most common and most likely answer to this crossword clue is the 15 letter word MOTHER-OF-PEARL
I've seen this crossword clue in Independent
M
O
T
H
E
R
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O
F
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P
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A
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L
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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 MOTHER-OF-PEARL mean?
MOTHER-OF-PEARL noun- The hard pearly inner layer of certain mollusk shells; nacre.
- A butterfly of the genus Salamis.
- Made from or looking like mother-of-pearl; iridescent or pearly.