It makes one sick (6)
I believe the answer is:
emetic
'it makes one sick' is the definition.
(an emetic medicine makes you sick)
This is all the clue.
(Other definitions for emetic that I've seen before include "Medicine designed to induce vomiting" , "Medicine taken to induce vomiting" , "Medicine taken to make one vomit" , "Something that causes vomiting" , "Medicine causing vomiting" .)