The most common and most likely answer to this crossword clue is the 9 letter word THICKENED
I've seen this crossword clue in Financial Times
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More clues leading to the result THICKENED
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 THICKENED mean?
THICKENED verb- To make thicker (in the sense of wider).
- To make thicker (in the sense of more viscous).
- To become thicker (in the sense of wider).
- To become thicker (in the sense of more viscous).
- To strengthen; to confirm.
- To make more frequent.