The most common and most likely answer to this crossword clue is the 8 letter word FRUCTOSE
I've seen this crossword clue in The Guardian
F
R
U
C
T
O
S
E
Buddy explains!

The straight part
We know that "sugar" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sugar = fructose.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 FRUCTOSE mean?
FRUCTOSE noun- A monosaccharide ketose sugar, formula C6H12O6.