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 (almost the whole word is kept).
'perfect' becomes 'ideal' (similar in meaning).
'ideal' with its last letter removed is 'IDEA'.
(Other definitions for idea that I've seen before include "Notion, concept" , "Notion, belief" , "Intention" , "a wheeze" , "Notion, image" .)