What's fatal for many — not opening best medicine? (8)
I believe the answer is:
laughter
'best medicine?' is the definition.
(ref to the saying 'laughter is the best medicine')
'what's fatal for many not opening' is the wordplay.
'what's fatal for many' becomes 'slaughter' (the killing of many people).
'not opening' means to remove the first letter (the opening letter is removed).
'slaughter' with its initial letter removed is 'LAUGHTER'.
(Other definitions for laughter that I've seen before include "Guffaws" , "Sound of amusement" , "Infectious thing" , "Hilarity" , "Sign of amusement" .)