Poison some drinking water (8)
I believe the answer is:
paraquat
'poison' is the definition.
(I've seen this before)
'some drinking water' is the wordplay.
'some' becomes 'part'.
'drinking' means one lot of letters goes inside another.
'water' becomes 'aqua' (I've seen this before).
'part' placed around 'aqua' is 'PARAQUAT'.
(Other definitions for paraquat that I've seen before include "Toxic substance" , "Toxic, fast-acting herbicide" , "Trademarked weedkiller" , "Poison" .)