Thought it up in time (6)
I believe the answer is:
notion
'thought' is the definition.
(notion is a kind of thought)
'it up in time' is the wordplay.
'up' is an anagram indicator.
'in' indicates putting letters inside.
'time' becomes 'noon' (noon is a kind of time**).
'it' is an anagram of 'ti'.
'ti' placed within 'noon' is 'NOTION'.
(Other definitions for notion that I've seen before include "Vague idea or opinion" , "desire" , "Idea, concept" , "A vague and possibly incorrect opinion" , "Vague understanding" .)