Unacceptable to claim inventor's first idea (6)
I believe the answer is:
notion
'idea' is the definition.
(notion is a kind of idea)
'unacceptable to claim inventor's first' is the wordplay.
'unacceptable' becomes 'not on' (something 'not on' is unacceptable or frowned upon).
'to claim' indicates putting letters inside.
'inventor's first' becomes 'i' (1st letter of 'inventor').
'noton' placed around 'i' is 'NOTION'.
(Other definitions for notion that I've seen before include "Vague or whimsical idea" , "Caprice" , "Idea or whim" , "Vague understanding" , "Vague or fanciful idea" .)