The most common and most likely answer to this crossword clue is the 9 letter word ACID-HEAD
I've seen this crossword clue in Financial Times
A
C
I
D
-
H
E
A
D
Buddy explains!
The straight part
We know that "drug user" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case drug user = acid-head.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.