Concept, belief (6)
I believe the answer is:
notion
This clue is a double definition.
'concept' is the first definition.
(notion is a kind of concept)
'belief' is the second definition.
(synonyms)
(Other definitions for notion that I've seen before include "Vague understanding" , "Vague impression" , "An idea or casual thought" , "Whimsical idea or small sewing article" , "Vague idea or opinion" .)