Almost perfect notion (4)
I believe the answer is:
idea
'notion' is the definition.
(both can mean a thought)
'almost perfect' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'perfect' becomes 'ideal' (similar in meaning).
'ideal' with its final letter taken off is 'IDEA'.
(Other definitions for idea that I've seen before include "Notion, concept" , "Notion, belief" , "Intention" , "a wheeze" , "Notion, image" .)