Plan that’s not quite perfect (4)
I believe the answer is:
idea
'plan' is the definition.
(thesaurus)
'not quite perfect' is the wordplay.
'not quite' means to remove the last letter.
'perfect' becomes 'ideal' ('ideal' can be similar in meaning to 'perfect').
'ideal' with its last letter removed is 'IDEA'.
'that's' is the link.
(Other definitions for idea that I've seen before include "Notion, image" , "Thought or notion (4)" , "Content of cognition" , "Suggestion" , "Thought, belief" .)