The most common and most likely answer to this crossword clue is the 14 letter word PHENOBARBITONE
I've seen this crossword clue in The Times
P
H
E
N
O
B
A
R
B
I
T
O
N
E
More clues leading to the result PHENOBARBITONE
Buddy explains!
The straight part
We know that "sedative" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sedative = phenobarbitone.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 PHENOBARBITONE mean?
PHENOBARBITONE noun- A barbiturate drug used as a sedative.