Return it in the middle of the day; just an idea (6)
I believe the answer is:
notion
'an idea' is the definition.
(I know that notion is a type of idea)
'return it in the middle of the day' is the wordplay.
'return' says the letters should be written backwards.
'in the' indicates putting letters inside.
'middle of the day' becomes 'noon' (I've seen this in another clue).
'it' inserted into 'noon' is 'noiton'.
'noiton' reversed gives 'NOTION'.
'just' is the link.
(Other definitions for notion that I've seen before include "Idea, belief" , "Idea or fancy" , "clue" , "An idea or casual thought" , "Caprice" .)