Not entirely perfect notion (4)
I believe the answer is:
idea
'notion' is the definition.
(thesaurus)
'not entirely perfect' is the wordplay.
'not entirely' means to remove the last letter.
'perfect' becomes 'ideal' ('ideal' can be similar in meaning to 'perfect').
'ideal' with its last letter taken off is 'IDEA'.
(Other definitions for idea that I've seen before include "Purpose" , "Aim" , "Theory" , "Mental impression" , "A suspicion of" .)