The most common and most likely answer to this crossword clue is the 10 letter word CARAMELISE
I've seen this crossword clue in The Guardian
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More clues leading to the result CARAMELISE
Buddy explains!

The straight part
We know that "treat with superficial sweetness" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case treat with superficial sweetness = caramelise.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 CARAMELISE mean?
CARAMELISE verb- To convert (sugar) into caramel.
- To brown (sugar) by means of heat.
- To undergo this kind of conversion or browning.