Idea: it arising in middle of day (6)
I believe the answer is:
notion
'idea' is the definition.
(I know that idea can be written as notion)
'it arising in middle of day' is the wordplay.
'arising' is a reversal indicator.
'in' is an insertion indicator.
'middle of day' becomes 'noon'.
'it' backwards is 'ti'.
'ti' placed inside 'noon' is 'NOTION'.
(Other definitions for notion that I've seen before include "Whimsical idea or small sewing article" , "Vague or whimsical idea" , "Idea, belief" , "Vague impression" , "Concept, belief" .)