Ailments kill some but not all (4)
I believe the answer is:
ills
'ailments' is the definition.
('ill' can be a synonym of 'ailment')
'kill some but not all' is the wordplay.
'not all' says the answer is hidden in the clue.
'ILLS' is hidden within 'kill some but'.
(Other definitions for ills that I've seen before include "complaints" , "Evils or sicknesses" , "Sicknesses or harms" , "Ailments, misfortunes" , "Sicknesses or misfortunes" .)