Concept that's less than perfect (4)
I believe the answer is:
idea
'concept' is the definition.
(I know that idea is a type of concept)
'less than perfect' is the wordplay.
'less than' means to remove the last letter (not the full word).
'perfect' becomes 'ideal' (similar in meaning).
'ideal' with its last letter taken away is 'IDEA'.
'that's' acts as a link.
(Other definitions for idea that I've seen before include "intellectual product" , "Notion in the mind" , "Thought, conception" , "Creative spark" , "Purpose" .)