The most common and most likely answer to this crossword clue is the 15 letter word POLYUNSATURATED
I've seen this crossword clue in The Times
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More clues leading to the result POLYUNSATURATED
Buddy explains!
The straight part
We know that "Sort of fat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sort of fat = polyunsaturated.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 POLYUNSATURATED mean?
POLYUNSATURATED adjective- Of or relating to long chain organic compounds that have multiple double bonds; polyunsaturated fatty acids are essential to human nutrition.
- (nutrition, of a fat or oil) Having a chemical structure that does not easily change into cholesterol (a substance containing a lot of fat though to be an important cause of heart disease).