The concept that it has gone back in time (6)
I believe the answer is:
notion
'the concept' is the definition.
(notion is a kind of concept)
'it has gone back in time' is the wordplay.
'has gone back' shows that the letters should be reversed in order.
'in' means one lot of letters goes inside another.
'time' becomes 'noon' (noon is a kind of time).
'it' reversed gives 'ti'.
'ti' placed within 'noon' is 'NOTION'.
'that' is the link.
(Other definitions for notion that I've seen before include "Vague or whimsical idea" , "Idea, belief" , "Vague or fanciful idea" , "A vague and possibly incorrect opinion" , "An idea or casual thought" .)