Notion's perfect, almost (4)
I believe the answer is:
idea
'notion's' is the definition.
(I've seen this before)
'perfect almost' is the wordplay.
'perfect' becomes 'ideal' (similar in meaning).
'almost' means to remove the last letter (most of the word but not all of it).
'ideal' with its last letter taken off is 'IDEA'.
(Other definitions for idea that I've seen before include "Notion, belief" , "Mental impression" , "intellectual product" , "Plan" , "Product of the mind" .)