The most common and most likely answer to this crossword clue is the 7 letter word THIAMIN
I've seen this crossword clue in The Daily Telegraph
T
H
I
A
M
I
N
Buddy explains!

The straight part
We know that "Organic substance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case organic substance = thiamin.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 THIAMIN mean?
THIAMIN noun- One of the constituents of vitamin B complex, found in meat, yeast and bran, that is necessary for the metabolism of carbohydrates.