The most common and most likely answer to this crossword clue is the 11 letter word MELLIFLUOUS
I've seen this crossword clue in Independent
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More clues leading to the result MELLIFLUOUS
Buddy explains!

The straight part
We know that "Sweet" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sweet = mellifluous.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 MELLIFLUOUS mean?
MELLIFLUOUS adjective- Flowing like honey.
- Sweet, smooth and musical; pleasant to hear (generally used of a person's voice, tone or writing style).