Poison in conspiracy an idea (7)
I believe the answer is:
cyanide
'poison' is the definition.
(cyanide is poisonous)
'in conspiracy an idea' is the wordplay.
'in' indicates a hidden word.
'CYANIDE' is hidden within 'conspiracy an idea'.
(Other definitions for cyanide that I've seen before include "killer" , "Extremely poisonous salt" , "Nice day (anag) - poison" , "Poisonous compound" , "Highly toxic substance" .)