The most common and most likely answer to this crossword clue is the 10 plus 5 letter word PEPPERMINT CREAM
I've seen this crossword clue in The Daily Telegraph
P
E
P
P
E
R
M
I
N
T
C
R
E
A
M
More clues leading to the result PEPPERMINT CREAM
Buddy explains!

The straight part
We know that "that’s sweet" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that’s sweet = peppermint cream.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.