It's said to be the best medicine (8)
I believe the answer is:
laughter
'it's said to be the best medicine' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for laughter that I've seen before include "Hilarity" , "Infectious thing" , "Manifestation of amusement" , "Cackling" , "Sign of amusement" .)