The most common and most likely answer to this crossword clue is the 9 letter word SPEEDBALL
I've seen this crossword clue in Independent
S
P
E
E
D
B
A
L
L
Buddy explains!

The straight part
We know that "mix of coke and H" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mix of coke and h = speedball.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 SPEEDBALL mean?
SPEEDBALL noun- A mix of heroin and cocaine.
- Coffee with espresso.
- A fastball.
- A competitive variant of paintball with an equal playing field, contrasted with woodsball.
- To occur fast, as if trying to finish as soon as possible.
- To take heroin and cocaine at the same time.