The most common and most likely answer to this crossword clue is the 13 letter word TRANQUILLIZER
I've seen this crossword clue in The Daily Telegraph
T
R
A
N
Q
U
I
L
L
I
Z
E
R
Buddy explains!

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 TRANQUILLIZER mean?
TRANQUILLIZER noun- That which tranquillizes or soothes.
- A drug used to reduce anxiety or tension; a sedative.