The most common and most likely answer to this crossword clue is the 11 plus 4 letter word UPSIDE-DOWN CAKE
I've seen this crossword clue in The Times
U
P
S
I
D
E
-
D
O
W
N
C
A
K
E
More clues leading to the result UPSIDE-DOWN CAKE
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 = upside-down cake.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.