Fancy it is served up during the middle of the day (6)
I believe the answer is:
notion
'fancy' is the definition.
(thesaurus)
'it is served up during the middle of the day' is the wordplay.
'is served up' says the letters should be written backwards.
'during' is an insertion indicator.
'the middle of the day' becomes 'noon' (I've seen this before).
'it' in reverse letter order is 'ti'.
'ti' going inside 'noon' is 'NOTION'.
(Other definitions for notion that I've seen before include "Idea or whim" , "Idea, concept" , "Vague understanding" , "clue" , "Vague or fanciful idea" .)