The most common and most likely answer to this crossword clue is the 9 letter word SMOOTHENS
I've seen this crossword clue in Financial Times
S
M
O
O
T
H
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S
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 SMOOTHENS mean?
SMOOTHENS verb- To make smooth.
- To become smooth.