The most common and most likely answer to this crossword clue is the 12 letter word ANAESTHETICS
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More clues leading to the result ANAESTHETICS
Buddy explains!
The straight part
We know that "Painkillers" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case painkillers = anaesthetics.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.
Crossword Answer definitions
ANAESTHETICS noun- A substance administered to reduce the perception of pain or to induce numbness for surgery and may render the recipient unconscious.