Thought it up in time (6)
I believe the answer is:
Here is my best explanation:
'thought' is the definition.
(notion is a kind of thought)
'it up in time' is the subsidiary indication.
'up' is an anagram indicator.
'in' is an insertion indicator.
'time' becomes 'noon' (noon is a kind of time**).
'it' anagrammed gives 'ti'.
'ti' put into 'noon' is 'notion'.
(Other definitions for notion that I've seen before include "An idea or casual thought", "clue", "Idea, concept", "A vague and possibly incorrect opinion", "Idea or whim", "Vague understanding", "Vague idea or opinion", "Vague or whimsical idea", "Belief, concept" and "Caprice".)