Butcher heading off for the best medicine (8)
I believe the answer is:
laughter
'the best medicine' is the definition.
(ref to the saying 'laughter is the best medicine')
'butcher heading off' is the wordplay.
'butcher' becomes 'slaughter' (synonyms).
'heading off' means to remove the first letter.
'slaughter' with its initial letter taken away is 'LAUGHTER'.
'for' is the link.
(Other definitions for laughter that I've seen before include "The expression of lively amusement" , "Manifestation of mirth" , "Sound of amusement" , "Sound of merriment" , "end of the comedian" .)