The most common and most likely answer to this crossword clue is the 3 letter word PUD
I've seen this crossword clue in The Times
P
U
D
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 = pud.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 PUD mean?
PUD noun- Pudding (either sweet or savoury).
- Penis.
- Child's hand; child's fist.
- An obsolete Russian unit of mass, equal to 40 Russian funt, or about 16.38 kg (approximately 36.11 pounds).
- A Russian unit of mass used for kettlebells, now rounded off to 16 kg (about 35.274 pounds).