Idea, notion (7)
I believe the answer is:
thought
I believe this clue is a double definition.
'idea' is the first definition.
(I know that idea can be written as thought)
'notion' is the second definition.
(I've seen this before)
(Other definitions for thought that I've seen before include "Pensee; consideration" , "Idea, concept" , "Supposed" , "Orwell's sort of police" , "Performed brain operations" .)