The most common and most likely answer to this crossword clue is the 6 plus 5 letter word CASTER SUGAR
I've seen this crossword clue in The Times
C
A
S
T
E
R
S
U
G
A
R
Buddy explains!
The straight part
We know that "Fine grains" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fine grains = caster sugar.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.