The most common and most likely answer to this crossword clue is the 6 letter word TANNIN
I've seen this crossword clue in The Times
T
A
N
N
I
N
More clues leading to the result TANNIN
- Not quite hiding the source of bitterness in wine
- Most striking stuff in certain wines
- Something from tea girl preserved?
- Can girl put in colouring ingredient?
- Feature of wine pub served up in function
- Something in wine bar turning brown outside
- It turns nasty at the outset, ringing a news matter in red
- Bitter substance girl put in can
- Girl comes into money, it's used in leather industry
Buddy explains!

The straight part
We know that "stuff in certain wines" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stuff in certain wines = tannin.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 TANNIN mean?
TANNIN noun- Tannic acid or any of its derivatives.