Thought it will turn up in time (6)
I believe the answer is:
notion
'thought' is the definition.
(I know that notion is a type of thought)
'it will turn up in time' is the wordplay.
'will turn up' says the letters should be written in reverse (in a down clue, letters go up).
'in' means one lot of letters goes inside another.
'time' becomes 'noon' (noon is a kind of time).
'it' back-to-front is 'ti'.
'ti' placed into 'noon' is 'NOTION'.
(Other definitions for notion that I've seen before include "Concept, belief" , "Whimsical idea or small sewing article" , "Idea or whim" , "Vague or whimsical idea" , "desire" .)