Cryptic crossword clue
Small case containing drug which will send you to sleep 8

The most common and most likely answer to this crossword clue is the 8 letter word SEDATIVE

I've seen this crossword clue in Financial Times
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Buddy explains!

The straight part
We know that "which will send you to sleep" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case which will send you to sleep = sedative.
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 SEDATIVE mean?

SEDATIVE noun
  1. An agent or drug that sedates, having a calming or soothing effect, or inducing sleep.
SEDATIVE adjective
  1. (pharmaceutical effect) Calming, soothing, inducing sleep, tranquilizing

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