The most common and most likely answer to this crossword clue is the 8 letter word DEMERARA
I've seen this crossword clue in Financial Times
D
E
M
E
R
A
R
A
More clues leading to the result DEMERARA
- . Deem inappropriate type of skirt to be something rather sweet
- Cook made to take back unusual type of sugar
- Sweet brown stuff from S American flower
- Rum old man hosts returning groups of soldiers
- Strange dream about time required to make sugar
- Ebb of a strange sea is something sweet
- Rolling Red Sea with a brown crystalline substance
- Editor spins in front of me with sort of skirt? That’s sweet!
- Rum made with rare ingredients
- A rare medic all but rejected sweetener
Buddy explains!

The straight part
We know that "brown crystalline substance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case brown crystalline substance = demerara.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 DEMERARA mean?
DEMERARA noun- A type of natural, unrefined or partially refined cane sugar, which is light brown in colour, particularly used in pastries and biscuits like shortbread.
- A dark rum, made in Guyana using molasses and this sugar, mainly used for blending.