Fancy an idea containing this, for example (6)
I believe the answer is:
poison
'fancy an idea containing this for example' is the definition.
The definition suggests a singular noun which matches the answer.
This is the entire clue.
(Other definitions for poison that I've seen before include "Toxic substance" , "Substance that damages tissues or causes death" , "Bane" , "I spoon up the lethal substance" , "Corrupt" .)