In the middle of the day, note an idea (6)
I believe the answer is:
notion
'idea' is the definition.
(notion is a kind of idea)
'in the middle of the day note' is the wordplay.
'in the' means one lot of letters goes inside another.
'middle of the day' becomes 'noon' (I've seen this before).
'note' becomes 'ti' (musical note, as in do re mi so fa la ti do).
'noon' enclosing 'ti' is 'NOTION'.
'an' is the link.
(Other definitions for notion that I've seen before include "Whim or fancy" , "Vague or whimsical idea" , "clue" , "Idea or whim" , "Idea or fancy" .)