The most common and most likely answer to this crossword clue is the 8 letter word NARCOTIC
I've seen this crossword clue in The Guardian
N
A
R
C
O
T
I
C
More clues leading to the result NARCOTIC
- Raced over bed when on enriched heart drug
- Numbing cold crushing Scott's heart, Amundsen ultimately coming first
- Crashed car into back of Merc, under influence of this?
- Inducing sleep at last, spin concentric circles?
- Went back to bed, one originally called number
- Painkiller used on Arctic trip
- Drug used on Arctic trip
- A number of disaffected Roman Catholics don’t use “shalom” in letters
- Drug needed on Arctic manoeuvres
- Anaesthetic with nitrogen and oxygen kept in cold
Buddy explains!

The straight part
We know that "Painkiller/Painkiller" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case painkiller/painkiller = narcotic.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 NARCOTIC mean?
NARCOTIC noun- Any substance or drug that reduces pain, induces sleep and may alter mood or behaviour; in some contexts, especially in reference to the opiates-and-opioids class, especially in reference to illegal drugs, and often both.
- Any type of numbing drug.
- Of, or relating to narcotics.
- (pharmaceutical effect) Inducing sleep; causing narcosis.