Notion, one almost gone (4)
I believe the answer is:
idea
'notion' is the definition.
(both can mean a thought)
'one almost gone' is the wordplay.
'one' becomes 'i' (Roman numeral).
'almost' means to remove the last letter (most of the word but not all of it).
'gone' becomes 'dead' (gone can mean deceased).
'dead' with its last letter removed is 'dea'.
'i'+'dea'='IDEA'
(Other definitions for idea that I've seen before include "Product of the mind" , "Light bulb above head" , "intellectual product" , "Proposal" , "Fancy" .)