Concept, idea (6)
I believe the answer is:
notion
This clue is a double definition.
'concept' is the first definition.
(I know that notion is a type of concept)
'idea' is the second definition.
(I know that notion is a type of idea)
(Other definitions for notion that I've seen before include "Idea or whim" , "desire" , "Idea, belief" , "Vague understanding" , "Idea, concept" .)