All the liquid is deadly (6)
I believe the answer is:
lethal
'is deadly' is the definition.
('lethal' can be a synonym of 'deadly')
'all the liquid' is the wordplay.
'liquid' indicates an anagram.
'all'+'the'='allthe'
'allthe' is an anagram of 'LETHAL'.
(Other definitions for lethal that I've seen before include "Deadly, potentially fatal" , "Murderous" , "describing a killer?" , "Killing" , "bringing down the curtain" .)