Idea not feasible, I gathered (6)
I believe the answer is:
notion
'idea' is the definition.
(notion is a kind of idea)
'not feasible i gathered' is the wordplay.
'feasible' becomes 'on'.
'gathered' means one lot of letters goes inside another.
'not'+'on'='noton'
'noton' enclosing 'i' is 'NOTION'.
(Other definitions for notion that I've seen before include "Vague idea or opinion" , "An idea or casual thought" , "Vague or whimsical idea" , "Whim or fancy" , "Vague impression" .)