Notion almost perfect (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 last letter removed is 'IDEA'.
(Other definitions for idea that I've seen before include "Product of the mind" , "Something imagined in the mind" , "It comes to mind" , "Purpose" , "Mental impression" .)