Vague idea (6)
'vague idea' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for notion that I've seen before include "Vague understanding" , "Vague or fanciful idea" , "Idea, concept" , "Whimsical idea or small sewing article" , "Idea or fancy" .)
(Other definitions for motion that I've seen before include "Formal proposal for debate" , "Moving - proposal" , "Moving - resolution" , "Gesture; formal proposal" , "inducing nausea in some travellers" .)