The most common and most likely answer to this crossword clue is the 9 letter word ANTIVIRAL
I've seen this crossword clue in Financial Times
A
N
T
I
V
I
R
A
L
Buddy explains!

The straight part
We know that "Drug" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case drug = antiviral.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 ANTIVIRAL mean?
ANTIVIRAL noun- An antiviral medication.
- (pharmaceutical effect) Inhibiting the growth and reproduction of a virus.
- Eliminating or inhibiting the action of a computer virus.