The most common and most likely answer to this crossword clue is the 12 letter word TRANQUILLISE
I've seen this crossword clue in The Daily Telegraph
T
R
A
N
Q
U
I
L
L
I
S
E
More clues leading to the result TRANQUILLISE
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 TRANQUILLISE mean?
TRANQUILLISE verb- To calm (a person or animal) or put them to sleep using a drug.
- To make (something or someone) tranquil.
- To become tranquil.