The most common and most likely answer to this crossword clue is the 9 letter word DISULFIDE
I've seen this crossword clue in Financial Times
D
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S
U
L
F
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D
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Buddy explains!
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 DISULFIDE mean?
DISULFIDE noun- A functional group with two sulfur atoms bonded to one another, described by the following formula: R–S–S–R'.
- The anion −S–S−.
- A binary compound of sulfur and another element in the ratio 2:1 (formula XS2).