The most common and most likely answer to this crossword clue is the 6 letter word OLEATE
I've seen this crossword clue in Financial Times
O
L
E
A
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More clues leading to the result OLEATE
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The straight part
We know that "Preparation of vegetable oil" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case preparation of vegetable oil = oleate.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 OLEATE mean?
OLEATE noun- Any salt or ester of oleic acid