As a concept, it turned up in the middle of the day (6)
I believe the answer is:
notion
'as a concept' is the definition.
I can't judge whether this definition defines the answer.
'it turned up in the middle of the day' is the wordplay.
'turned up' says the letters should be written backwards.
'in the' is an insertion indicator.
'middle of the day' becomes 'noon' (I've seen this before).
'it' back-to-front is 'ti'.
'ti' inserted into 'noon' is 'NOTION'.
Can you help me to learn more?
(Other definitions for notion that I've seen before include "Idea or fancy" , "A vague and possibly incorrect opinion" , "Vague idea or opinion" , "desire" , "Vague or whimsical idea" .)