Notion, thought (4)
I believe the answer is:
I believe this clue is a double definition.
'notion' is the first definition.
(both can mean a thought)
'thought' is the second definition.
(idea is a kind of thought)
(Other definitions for idea that I've seen before include "Plan" , "Product of the mind" , "Fancy" , "Flash of inspiration?" , "Light bulb above head" .)