Flipping idiot drinking neat poison (5)
I believe the answer is:
toxin
'poison' is the definition.
(toxin is a kind of poison)
'flipping idiot drinking neat' is the wordplay.
'flipping' is a reversal indicator.
'idiot' becomes 'nit' (I've seen this before).
'drinking' indicates putting letters inside.
'neat' becomes 'ox' (I've seen this in other clues).
'nit' backwards is 'tin'.
'tin' enclosing 'ox' is 'TOXIN'.
(Other definitions for toxin that I've seen before include "Poisonous substance" , "Form of poison" , "Poison of organic origin" , "Natural poison" , "Poison, venom" .)