All the mixture is deadly (6)
I believe the answer is:
lethal
'is deadly' is the definition.
(synonyms)
'all the mixture' is the wordplay.
'mixture' indicates an anagram.
'all'+'the'='allthe'
'allthe' is an anagram of 'LETHAL'.
(Other definitions for lethal that I've seen before include "very nasty indeed" , "Vicious" , "Killing" , "Liable to cause death" , "bringing about the end?" .)